Two impressive painters at opposite ends of the art

Richard Diebenkorn is a phenomenon in American art. He has moved, in his paintings, from an Edward Hopper-like representationalism, through a highly personal form of abstract-expressionism - a bold and chunky form of figurative art - to the creation of some of the most beautiful and elegant abstractions of recent years.

And he has now, as evidenced by the current exhibition of his most recent works at M. Knoedler & Co. here, moved ahead once again into new and fertile territory.

He has done so by placing a greater emphasis upon the decorative potential of his imagery, and by allowing color to play a more blunt and aggressive role in his work than it has of late.

Diebenkorn's recent paintings, known collectively as the ''Ocean Park Series, '' are among the most exquisite produced anywhere during the past two decades. They are generally large, muted in color - but with occasional flashes of bright color - geometric, and flat in appearance. They don't, at first, look very impressive, but in their own quiet and unobtrusive way, they have managed to prove that Diebenkorn possesses a first-rate painterly intelligence and sensibility.

The works in this series represent an extraordinary fusion of elements of various 20th-century modernist discoveries - from cubist space to Matisse's manipulation of form through color - with Diebenkorn's own abstract-expressionistic, coloristic, free-form, and representational explorations. Each of these paintings, as a result, is not only a handsome and tantalizing piece of painting, but is also a living and dynamic part of today's continuing modernist dialogue.

That ''dialogue'' shifts a bit with this exhibition - toward a more compact imagery. The works included in it are color-drawings and studies and are preliminary steps in a new direction. They are relatively small, were executed in gouache and crayon, and are unusually opulent. In fact, there is a ''ripeness'' about some of their forms that suggests the late paper cutouts of Matisse. And yet close study reveals that these images, decorative and even static as some of them may appear, are logical progressions from his previous work.

The problem is that Diebenkorn is also a highly intuitive painter - a fact that tends to disrupt the neat balance of his creative sensibility whenever he is probing into new areas. The result, at such times, is work that is often more tentative and promising than it is focused and accomplished. Judging from this exhibition, this apparently is one of those times, and the works on view are the products of such a period.

Although a good dozen or so of the pictures are as good as anything he has ever done - and ''Untitled No. 34,'' and ''Untitled No. 50'' are heartbreakingly beautiful - the show as a whole tends to be a bit spotty. It exists more as a promise of new and exciting things to come than as an exhibition of fully realized art.

This, however, doesn't bother me in the least. Not only because what lies beyond demands such probings and possible false starts, but also because I have never yet known Diebenkorn ultimately not to come through with flying colors.

This extremely fascinating and valuable show will remain on view at M. Knoedler & Co. through Jan. 28. Gregory Gillespie

Gregory Gillespie is a painter at the very opposite end of the painterly spectrum from Diebenkorn. He is precisely realistic, with a remarkable flair for drawing, and with a satiric touch that verges, at times, on the grotesque.

In his current exhibition at the Forum Gallery here, he proves once again that he is also a rugged individualist, a painter/draftsman who continues to create highly personalized and emotion-charged ''realistic'' paintings in a time when most realist art is either frigidly impersonal or photographically precise - or consists of vaguely sentimentalized renderings of banal subjects or events.

Gillespie is nothing if not highly individualistic, even idiosyncratic. There is a sharp-eyed clarity and warmth of tone to his pictures that reminds me somewhat of early Flemish painting - but of Flemish painting updated to confront 20th-century ideas, forms, and realities - and to articulate a very modern sense of Angst. Gillespie is a realist with a very tart and focused point of view - and seems alternately to be on the verge of painting imps and demons - or a wonderfully exotic Utopia.

This latter aspect of his art is particularly manifest in two smallish landscapes that can only be described as ravishing and ideal - in a slightly melancholy and pre-Raphaelite sort of way. They are among the most clearly realized and best-painted landscapes I have seen in quite a while and are a welcome antidote to the hundreds of landscapes on view in the galleries these days that resemble nothing so much as blown-up color transparencies. Barbara Valenta

Barbara Valenta is an artist with an excellent current exhibition at the Betty Parsons Gallery here. Her constructions of wood and cloth (as well as a handful of drawings) represent a considerable step forward from her last show two years ago, and indicate that a strong new voice is in the making. Her ability to redefine spatial environment by means of linear and planar thrusts and counterthrusts is particularly impressive - as are her skills at drawing.