Retired executives volunteer for nonprofit organizations.

Many top-level executives don't want to put their know-how on the shelf the minute they retire. They are eager to go on solving tough problems, working their way through difficult financial situations, and reaching for new ideas.

That's where the National Executive Service Corps comes in. This five-year-old counseling organization uses retired executive men and women who want to volunteer their time and expertise to act as consultants to America's thousands of nonprofit organizations.

These active retirees donate their consulting services on a part-time basis for specific projects that usually last from three to six months. They monitor the projects and later evaluate them. Their work is free, except for work expenses. A modest and negotiable fee is collected from the organizations seeking their help, which goes toward the maintenance of a headquarters staff and office at 622 Third Avenue in New York City.

Through the corps, E. V. Blissard, a computer expert who was staff systems analyst at Shell Oil Company in Houston until his retirement, volunteers his services to the Southern University System of Baton Rouge, La. With his help, several small black colleges in Louisiana were connected by a new computer system.

Mrs. Marian S. Heiskell of New York City, a retired newspaper executive and civic worker who still serves on many boards, including the New York Times, volunteers her services as a member of a panel of experts for Outward Bound. Among other things, the panel checks the organization's safety precautions.

John W. Queenan, retired managing partner of Haskins & Sells in New York, helps the Lutheran Church in America review its forecasting, budgeting, and planning procedures. J. Henry Smith, retired Chairman of Equitable Life, chairs a four-man volunteer committee to develop a more effective relationship between the New York Chamber of Commerce and the Economic Development Council.

Today there are some 800,000 nonprofit organizations in the United States operating in the fields of health, education, social services, the arts, and religion. Most of them depend on the help of some 80 million volunteers a year. Many of them are in need of expert guidance on how to improve management and operational efficiency, raise funds, or make long-range plans.

This year, in New York City alone, volunteer executives are expected to contribute over 15,000 hours (estimated in value at over .5 million) to over 40 projects. Their assistance has been sought by a variety of groups, including the New York Botanical Garden, the National Council on Crime and Delinquency, Girls Clubs of America, and the Agency for Child Development.

The National Executive Service Corps was founded in 1977 by Frank Pace Jr., who is its chairman and chief executive officer. It was begun with a $300,000 grant from the Edna McConnell Clark Foundation.

Mr. Pace, looking for a way to offer good business judgement and management assistance to the nonprofit organizations, many of them facing more serious financial problems than ever before, patterned the new domestic corps after the International Executive Service Corps he founded 18 years ago. That organization offers the managerial skills of retired US executives to government and corporation projects in 71 countries and has government funding.

US executive corps sponsors are all from the private sector. They include corporations, foundations, and individuals.

Fred Richards, vice-president in charge of recruiting for the corps, matches individual skills and expertise to the needs of specific organizations asking for help. Almost 40 percent of the 425 volunteers in New York City have been chairmen, presidents, or senior vice-presidents of Fortune 500 companies, Mr. Richards says.

Today, they are generously giving away their knowledge and experience to help the ''nonprofits'' by consulting on organizational restructuring, management practices, policies, systems, marketing, computerization, and financial planning. Although most of the executive volunteers come from business, many also come from the academic world and from government service.

Speaking of projects already completed, H. E. Alleman Jr., director of coordination of the National Executive Service Corps, mentions two examples among many: ''Our very first project in New York City was to help the Museum of the American Indian develop a plan for finding a more accessible location for showing its extensive collection of artifacts. We have helped Howard University's Divinity School develop a strategic plan for raising funds.''

The executive corps in New York has helped spawn similar organizations in 12 other cities: Boston; Rochester, N.Y.; Tampa, Fla.; Indianapolis; Detroit; Chicago; Denver; Minneapolis; Little Rock, Ark.; Dallas, Los Angeles, and San Francisco. Development work has begun to establish others in Philadelphia; St. Louis; Wichita, Kan.; and Houston.

Mr. Richards says the corps would like to see local organizations in 20 cities by 1985. These autonomous local groups are loosely amalgamated by an organization called the Managerial Advisory Service, which holds annual meetings for the exchange of information and ideas.