A new role for the first lady of American theater

With her feet propped comfortably on her rose-patterned couch, the first lady of the American theater crinkles her nose and tips her head back in the kind of lilting, girlish laugh that's been delighting audiences for more than 70 years.

She has a leprechaun's flair for storytelling and some of her favorite stories are those she tells on herself. She's just recalled a recent visit to the US Senate, where she was asked to speak on behalf of proposed legislation for home care for the elderly.

''There I was on the floor of the Senate at age 82, making a big noise and telling everyone that my generation has got to be noticed,'' she says, dabbing at a merry tear. ''Later on I thought, 'You've got to watch yourself and be careful that you don't get too demanding or too cocky!' ''

Helen Hayes, who has reigned on Broadway as Victoria Regina and Mary, Queen of Scots, is playing a new leadership role these days. Somehow, she says, she's ''sort of drifted'' into being a spokeswoman for her generation. Not for the aging or the elderly or even for senior citizens - ''I hate all those words'' - but for what she calls ''grandpersons.''

''People used to think that our minds stopped functioning abruptly at 50, but they seem to be more trusting of us today,'' she says with an emphatic wink. ''Of course, I may feel that way because I happen to travel in a circle that has great respect for age. In the theater, wonderful roles are written for character actors and people never have to give up as they grow older. It's not like professional football.''

At a time when 1 out of every 5 Americans is 55 or older, it's not surprising that society's attitudes toward this fastest-growing segment of the US population are changing. What is surprising, Miss Hayes says, is how the older generation's attitudes toward themselves are changing.

''There are plenty of people in my age group who are so pleasantly in tune with life and the times,'' she continues. ''I think you hear far fewer of us saying, 'Oh, but in my day. . . .' And I think that's because we enjoy the world more than a lot of younger people who seem to be overwhelmed by it.

''I love the world today! I wouldn't have lived in any other time, and I can't tell you how many of my contemporaries say the same thing.''

Gazing around the cozy living room, where pink and white roses flourish on every Victorian chair in sight and freshly cut perennials fairly burst from surrounding vases, one might be tempted to think that the sentiments are those of a dewy-eyed optimist. But the large oil portrait of her daughter, Mary, that hangs on one wall and the bookshelf copy of ''Charlie,'' a biography of her playwright husband Charles MacArthur, who passed on soon after Mary's untimely death, are two poignant reminders that life has not been all roses and laughter for Helen Hayes.

When she talks about the reserves of courage and irrepressible humor which have helped her long-time friend Mary Martin through some difficult times, she could be speaking of her own approach to life's challenges: ''Her philosophy . . . which she developed at a time of a great personal loss, is not to indulge her sadness. I find that a very powerful admonition. So much of life and so many relationships are governed by this kind of negative indulgence. It is an attitude against which all of us must guard.''

Those remarks come from a broadcast Miss Hayes did for ''The Best Years,'' a two-minute radio commentary for ''grandpersons'' that is now aired on more than 150 stations nationwide. She took over the program when Lowell Thomas passed on last year, and she says it's become one of her most enjoyable jobs.

In addition to her legendary work on behalf of the Actor's Fund and the time she volunteers each week at a local hospital named in her honor, Helen Hayes is on practically everyone's list for chairmanships and benefits. Pointing to a recent gift from some friends, a gold lapel button that reads ''No,'' she concedes that ''Word has gotten around that I don't know how to say 'No.' Everybody seems to know it.''

When she's not hosting a benefit for Soviet Jewish pianist Vladimir Feltzman, she's flying to San Francisco to attend a party for actress Ina Claire, or hopping a jet to visit friends in England. On one recent trip to London, she spent a typical day touring stately homes, visiting Queen Mary's rose garden, dropping by the Tate Gallery to see the Turner collection, and dining with former President Gerald Ford. ''Sipping Perrier water, surrounded by Secret Service men, gave me a real thrill!''

Another of her great joys is keeping in touch with friends, many of whom she's known for 40 or 50 years. Lillian Gish is among the houseguests who come to Nyack for Christmas every year, and Miss Hayes recently invited 65 other close friends for a cruise on the Hudson River to celebrate her birthday.

''I told the 65 people who were on that boat that they were there because I felt a special warmth and devotion and affection for them,'' she says. ''There wasn't a soul on board who I thought could help me get ahead, not a soul that I owed a debt to. Everyone was there because I loved them and because they were my friends - and that's an awful lot of people to keep on trying to be close to!''

Although her son, actor James MacArthur, now lives in Hawaii, he and his family are frequent visitors at the unpretentious Queen Anne home overlooking the Hudson. She regrets being an ''absentee grandmother'' for his children, but her eyes sparkle with affection when she talks about her relationship with her son and the fun they've had appearing together on such television shows as ''Hawaii Five-O'' and ''Love Boat.''

''I'll go on any show that asks me to appear with Jim,'' she says. ''He's such a delight to work with, and he's very much beloved. All through the business of being a television celebrity and being highly admired, he's carried a great urge to serve and to please. He really yearns to do for people, and I think that's a most gratifying trait.''

Where did he learn such caring?

''I'm not sure . . . that it's something you're born with . . . I think perhaps he got it here . . . in this house.''

The pauses are unstaged and well up from the kind of sincerity that playwrights can't manipulate and directors can't teach. To listen to Miss Hayes' gentle observations about marriage and family is to sit at a favorite grandmother's knee, drinking in stories of wonderfully simple wisdom.

''I know too many young women today who are desperately searching for themselves, and who have tossed a lot aside to do so. I suppose this business of being in search of yourself is all right, but not to the exclusion of other things, of your duty or your obligations to other people - like spouses and children.

''I think you can be a free spirit and still be a giving one, and marriage is one long giving on both sides. I don't think there's enough of that today.''

In two of her autobiographical reminiscences (''On Reflection'' and ''A Gift of Joy''), there is the recurring thread of her own feelings of inadequacy in her marriage to the flamboyant Charles MacArthur. But she appears to have resolved those doubts today, as she talks about the years they shared.

''Charlie had an extraordinary mind; he was a great talker, a very gregarious person, and he loved to be with people,'' she recalls. ''Often, in retrospect, I've felt that I wasn't enough for Charlie. But then I say to myself, 'I was the object of his love, and that made me enough for him.' I console myself that I gave him an object to pour his love upon.

''He had a tremendous capacity to love, which is a very great gift, and which comes only to people with exceptional minds. It's just stupid people who don't know how to love.''

As she looks ahead to the future, Helen Hayes says she's reminded of a favorite verse from the popular song ''Moon River.''

''There really is 'such a lot of world to see,' don't you think? There's such a lot to study and explore - and I don't mean in the physical world.''