More than 500,000 people each year file their way into the Medici Chapel, a 16th -century addition to the older Florentine Basilica of San Lorenzo. Though there is a small altar in the chapel's apse, the architect, Michelangelo Buonarroti, built the chapel primarily to be a mausoleum for two members of the Medici family, Lorenzo II, who died in 1519, and Giuliano, a son of Pope Clement VII, who died in 1516.

Michelangelo's two sculptured tombs for the men he referred to, perhaps sarcastically, as his "two captains," are masterpieces, which is why so many people buy their tickets and queue to enter the tiny chapel.

The trouble has been that both the entrance and the exit are in the same narrow passageway, and in high season there has been a bit of a crush. A 42-year-old Florentine, Paolo Dal Poggetto, who was named director of the chapel-museum in 1970, decided to see if another exit could be found. What be found, instead, were 180 unknown sketches on walls.

Dal Poggetto claims, and he has not been seriously challenged, that 97 of the sketches are by Michelangelo's own hand - 54 of them figurative, 43 architectural. There are other interesting drawings as well, believed to be by the master's students or assistants.

This discovery means that the tourists will have to find another way out of the chapel. It also means that if they wish to see the wall sketches they must put their names on a list as most of them are in a cramped cellar-like room and only 12 people are allowed in every half hour. The only extant drawings by Michelangelo prior to this discovery in 1976 (but only now open to the public) are all on paper and not more than 20 inches in size. Some of the newly-discovered drawings are larger than life-size.

In his search for the new exit, Dal Poggetto removed old wardrobes and cabinets from a small vestry off the chapel. Beneath one wardrobe he found a trapdoor and a flight of stairs that lead to a small rectangular room which is cellar-like, but is actually a few inches above street level. It contained a well, some lumber, and several centuries of dust.

The storeroom had last been used about 20 years ago when the chapel's custodians kept charcoal there for the braziers they used for heating. When the rest of San Lorenzo was cleaned of mud and debris from the 1966 flood, the hidden room was not cleaned. It had simply been forgotten.

Acting on an educated hunch, Dal Poggetto decided the first thing to do was to remove the whitewash from the storeroom's walls. This was done slowly, painstakingly, by using a surgeon's scalpel. There were two layers of whitewash of different periods that had to be removed to reach the original wall.

There they found sketches by Michelangelo, in charcoal and in siennared crayon. Some are clearly early sketches for the master's works then in progress, others are of works commissioned but not yet started. One sketch seems to be the dawning of an idea which was to be executed much later on the walls of the Sistine Chapel in Rome.

Michelangelo destroyed most of his own sketches and drawings, he was never known to have done experimental drawings on any wall. Why then would he have chosen to "practice" on those part-cellar walls and leave behind a wealth of immortal doodlings?

Dal Poggetto's answer to that seems sound, and if it could be proved, which is most unlikely, it would, in itself, make a fascinating footnote to history.

It is known that in the year 1530, Michelangelo was working on the Medici tombs in the San Lorenzo church. When it became clear that Florence soon would be attacked by the papal and imperial armies, the artist-artisan was told to see to the fortification of the city.

When the Florentine republic fell in August 1530 the troops were told to arrest Michelangelo and lock him in the Bargello, then the city prison (and today a museum which contains several of his sculptures). He went into hiding.

Dal Poggetto thinks he hid in the storeroom beneath the trapdoor and stayed there about two months. The drawings here, then, are the graffiti of a prisoner or fugitive from papal ire.

There are several sketches on the wall of the Laurentian Library which the artist was later to design. There are studies (one of them 6 feet tall) of the already commissioned painting of Leda and the Swan. There appears to be an idea for another statue of David, emerging from the artist's brain, and which later was done, and is sometimes also called his Apollo. There is a sketch of a risen Christ which is nearly 7 feet tall.

The most impressive single drawing is that of a pair of virile legs - which also were later to be executed in marble, slightly altered from the sketch. They probably are the most famous marble legs in the world - those of the statue of Giuliano De' Medici, over his tomb in the chapel.

The first drawing to come to light was seen on March 22, 1976. On approximately that same date, this year, full-sized photographic reproductions of all the drawings were unveiled in the museum of the Forest Lawn Cemetery in Hollywood, Calif., the graveyard of the stars.

That remarkable place already has a full-scale replica of Michelangelo's giant David, and a mosaic reproduction of Leonardo's Last Supper fresco, and other Renaissance masterpieces worthy of such hallowed ground. Dal Poggetto says he is pleased by such rapid recognition of what he found in looking for a new way out. CAPTION: Sketch, A portion of Michelangelo's sketches in the Medici Chapel. AP