PHILIP JOHNSON's country estate is a self-portrait in architecture unequaled since Thomas Jefferson's Monticello.

The estate is also a three- dimensional history of architecture in the last half of the 20th century.

A study is the seventh and most recent structure to be built on Johnson's 35-odd-acre estate in New Canaan, Conn., just outside Stanford. Johnson has agreed to leave the land and its buildings at his death to the National Trust for Historic Preservation, with a substantial endowment for upkeep.

In any case, Johnson, at 75, is going strong, with a second wind since he had open heart surgery at 70.

In recent years he has had a great burst of creation, designing one remarkable building after another, with his partner, John Burgee. He has won the American Institute of Architects' highest honor, the gold medal, as well as numerous other awards, including the Alcoa award, and fittingly, the University of Virginia's Thomas Jefferson award (though Johnson says he's not that fond of Monticello).

Among his best known buildings are the Fort Worth Water Gardens, the Crystal Cathedral in Garden Grove, Calif., and in New York City, the New York State Theater at Lincoln Center, the Seagram Building (with Mies van der Rohe) and the controversial AT&T office building now being completed with what has been described as a Chippendale top.

Not long ago, Johnson gave what he called a "Vernissage" a term used for the opening of a painting exhibit, so called because once an artist's friends would come and put the varnish on the paintings the night before a show opened.

Johnson's party was to show off his study. Many of the most important architects in the country were there. They came not only because Johnson has won all the major architecture awards, but also because he may be the world's greatest architecture enthusiast. He is almost as interested in other people's work as he is in his own. Many younger architects have profited from his support.

Before World War II, Johnson wrote (with Henry Russell Hitchcock) "The International Style," which introduced Marcel Breuer, Mies van der Rohe, Le Corbusier and other European architects to Americans. When the Museum of Modern Art was opened in New York City, he not only founded its architecture department, but also paid the staffhs salaries. He has been one of the one or two largest donors (several million dollars in paintings) to the Modern.

At the height of his popularity as an architectural historian and critic, when he was in his mid-30s, he chucked it all and went back to Harvard (where he'd originally majored in philosophy and minored in Greek) to study with Brauer and earn an architectural degree.

Today, he still remains the most articulate spokesman for architecture.

On his estate, his buildings speak for him. They show not only his development as an architect, but the way architecture and the world have changed. For him, the buildings have been like maquettes for larger structures; for everyone else, they are like a time line of architecture of the past 32 years.

The estate came with a building at the street level, a pleasant enough dark-brown shingled house.

His first building on the estate, in 1949, was a small brick rectangle, called the guest house. Three sides, like the new study, were closed and secret; the fourth side looked at the world through large portholes.

Inside, the building is divided into two principal lush rooms with heavy white carpeting and walls of dark cork. One room has a bed positioned underneath a series of arches. (The arches appear again in other buildings.) Sliding panels close off the window and the door, so you feel wrapped in a delicious cocoon.

The other room is a sitting room, full of non-architecture books and two comfortable leather Charles Eames chairs. The other day "Gorky Park" was on the coffee table. The house also has a bath, a dressing room and a small kitchen, as well as an entrance hall, with, of all things, two doors that aren't real, designed for symmetry.

The second building is the glass house, generally considered Johnson's masterpiece, though he concedes it was influenced by Mies van der Robe. The glass house is the most exciting yet satisfying building I've ever been in.

The house is a rectangle, a single room, 56 feet by 22 feet and 10 1/2 feet high. A brick pillar holds the fireplace on one side, the bath on the other. A wooden cabinet, just about head high, serves for storage and as a headboard. A lower cabinet, counter high, contains the sink, stove, refrigerator, garbage can and towel rack.

The furniture is almost all very early examples of designs by Miss van der Rohe and Lilly Reich, commissioned by Johnson in the '30s for his New York apartment. In the last year, Johnson has taken great pains to put everything in order. He has had the Barcelona chairs and the Mies chaise longue rechromed and reupholstered. All the wooden cabinets are new. The marble dining table has been repolished. In the bath, the pigskin tiles on the wall have been replaced ("they rotted," said Johnson), after more than 25 years, with tile. Many trees have been removed so the buildings can be better seen.

At Johnson's party, Cesar Pelli, the architect, recalled the time he changed his shirt in the bath. "I took a stray thread off the shirt, and put it on the wash bsin. Johnson, who was passing by, picked it up immediately and put it in the trash." John Tutun, his gardener for 22 years, says Johnson picks up cigarette butts in the lawn.

Pelli likes to tell about breakfast in the glass house with Johnson. "We were eating sardines, and I said he really needed rectangular sardine cans to match the house. He said with enthusiasm, 'Where can I buy them?'"

At a dinner, Pelli remembered, the caterer arrived with glass tables after the company was assembled. After dinner, chairs were arranged on the west side facing the glass house, and everyone enjoyed the sunset reflected in the glass wall. "It was gorgeous," said Pelli. "The house's structure framed the sunset."

In 1962, Johnson, who has said "we cannot not history," built his third structure, an arched pavilion, on the lake. The folly is a summer house, just six feet high, so that from the glass house it looks much farther away, another visual trick.

In 1965, after he had amassed a great collection of contemporary paintings on the advice of his friend, David Whitney, a museum consultant, Johnson built a forward-thinking, underground, dirt-bermed gallery, set into the hill to the northeast of the house. The building predates most of the current enthusiasm for the energy efficiency of underground buildings. The controlled light also helps preserve the paintings. All you see from the outside is the mound and the dark-red sandstone, pi-shaped entry. Inside are three carrousels, 13 feet high. Tracks hold panels on which the paintings are mounted. There is a small kitchen and bath.

The 1970 building is a sculpture gallery, to the northwest of the painting gallery. Johnson once said of the building, "You can't draw or photograph its geometry." He has said it may be the "most original, the freest of all the structures." Others say it is the prototype for his crystal cathedral in California. The walls are solid brick with no openings except for a service door on the lower level and the entry door. The roof is all glass, defined by light tubes and metal frame. The six levels of the building are transversed with a ceremonial stair. You walk down, feeling as though you're going to your coronation (or beheading), at last to find yourself at the mystery -- a dark, secret level, unseen from above.

Johnson says that, for the moment, he has no plans for any other building on the property. But his friends and lovers of architecture hope he won't stop now.