In his last years, Picasso took the leash off his genius

Picasso's genius first manifested itself in his early teens, and remained with him until the very end. During all that time - he lived to be 92 - that genius kept him perpetually on edge, perpetually seeking new and better ways to give form and expression to the extraordinary life-force that bubbled within him.

His genius gave birth to the elongated and tragic figures of his Blue Period, to the slightly more relaxed and elegant images of his Rose Period, and then to his truly magnificent Cubist canvases, drawings, sculptures, and prints.

But that was only the beginning. His career still had more than 50 years to go. Years which saw an endless stream of works leave his hand, from dozens of sculptures, hundreds of prints, and thousands of drawings, to such masterworks as ''The Pipes of Pan,'' ''Girl Before Mirror,'' and ''Guernica.'' And which then, during the last decade of his life ending in 1973, saw an explosion of hundreds of passionately alive and inventive canvases that not only summarized his work to date, but in some instances actually leaped beyond it.

Until recently, however, the art world and the public didn't view his late works so favorably. They were seen as weak recapitulations of earlier paintings, or as unwitting parodies of what he had produced during his prime. Even as recently as 1981, much more criticism than approval was expressed during the Pace Gallery's exhibition of several of his very last works.

Some of that is understandable. Picasso did spew forth such a volume of work those last years that much of it quite naturally fell below his high standards. But enough magnificent canvases and prints were produced during that period to fully justify not only a critical reappraisal of his late work, but a major exhibition devoted to it as well.

''Picasso: The Last Years, 1963-1973,'' currently at the Guggenheim Museum here, is just such an exhibition. Its roughly 200 paintings, drawings, and prints include some of the most self-revelatory and controversial works of his career, and establish beyond doubt that his genius did indeed remain with him until the very end.

Except for the antiwar series, ''Rape of the Sabines,'' which he completed in early 1963, these late works focus more on areas of individual concern than on broadly-based social or political issues. He was particularly drawn to the theme of ''The Artist and his Model,'' and returned to it time and time again. But mostly, these last works probe in any direction that struck his fancy. There are portraits and figure studies; voluptuous and satirical nudes; witty takeoffs on Old Master paintings; depictions of serious and profane love; still-lifes and landscapes; allegories - and almost everything else.

All this is dashed off with great verve and flair, and with the gusto of a very young child attacking paint and paper without concern for rules or limitations. The only thing that mattered was getting what he felt, saw, or wanted to express out onto canvas or paper. All rules went out the window. He was free to do whatever he wanted and in whatever form he wanted. He had one great advantage over anyone else dreaming of doing the same thing, however: several decades of experience as one of the most brilliant and innovative artists of all time and a magnificent body of work that assured him a significant position in art history no matter what these last pictures looked like.

As noted above, some of these late paintings didn't turn out too well. But I doubt he cared. If one failed at 11 a.m., he'd start another a few minutes later , and then possibly do another later in the day. His production during this period was prodigious: An exhibition held in Avignon in 1973 consisted of 201 large paintings produced over a 20-month period.

Most, if not all, I'm certain, were done at high speed with little, if any, formal or technical self-consciousness. As a result, it is pointless to analyze these works in strictly formal terms. After 60-some years of painting and drawing, it simply makes no sense to believe he would consciously decide to place a particularly subtle Cubist device next to a particularly effective DaDa gimmick, and beside a motif taken from one of his classical works. He no more needed to do that than he needed to remind himself that mixing blue and yellow produces green.

No, all that was done impulsively and ''automatically,'' and with the accrued skill of a master chef mixing the ingredients for his specialty. Such a chef needs no recipe, and neither did Picasso: He could spend his last years cooking up a storm. He could mix new ingredients to see what exotic dishes he could concoct, or he could dump together elements he previously would never have dreamed of combining in order to find new and interesting combinations. But whatever, the point was to remain open to everything, not to fear something merely because it had never been done before, or because doing it violated certain taboos.

Everything was not fun and games, however. He wouldn't have been Picasso if he hadn't also had his solemn, anxious, and deeply serious moments - and if all that hadn't come out in his art.

This is particularly apparent in his June 30, 1972 ''Self-Portrait.'' It depicts the same dark-eyed Picasso we've seen in countless other self-portraits and photographs. But this time there's a difference. All the cocky self-assurance is gone - as are all hints that we are face to face with a genius. What we have left is an image of a very vulnerable and mortal man fully aware of the brevity of human - but most particularly his own - life. And yet there is no suggestion of fear or cringing, or any wish to evade the truth. As always, Picasso knew precisely who he was and what was confronting him. And like Rembrandt before him, he transformed that moment of awesome self-awareness into a stark and haunting work of art.

At the Guggenheim Museum through May 6.