In Paul Mellon's house in Washington, down a narrow corridor, far from public sight, hangs a reed-pen van Gogh drawing of a blue cart in a field. Near it hangs another far rougher work of art. The second drawing is prophetic. It shows two trees and a meadow and a four-legged grazing animal, almost certainly a horse. It was drawn on Nov. 21, 1914, when the artist -- Paul Mellon -- was 7.

The calm of that small picture, its pastoral tranquility, is to some degree misleading. Much of Mellon's childhood had been spent in turmoil. In 1912, after tearful scenes of enmity amid reporters and detectives, his parents had been granted a divorce.

If Mellon late in life seems a whole and fulfilled man, some portion of the credit must be given to the pleasure, to the solace, he has taken from the horse.

Tonight, at a white-tie dinner at the National Gallery of Art -- after 47 years of serving the museum founded by his father, Mellon, 77, will announce that he is stepping down as chairman of the board. His departure ends an era. Two exhibitions there -- one of Leonardo drawings from Queen Elizabeth's collection, the other of the paintings of the Englishman George Stubbs -- have been mounted in his honor. It is entirely appropriate that both pay homage to the horse.

"I've had a lot of fun on top of them," says Mellon. "They've been my one great recreation." He breeds them and he ponders them and, until a few years ago, jumping hedges and stone fences, he rode them to the hounds.

When the Yale Center for British Art and British Studies, the $150 million fully stocked museum he gave his alma mater, opened to the public in 1977, the donor was not there. He was riding a horse named Christmas Goose in a three-day competition, a timed 100-mile trail ride through the mountains of Virginia. He has described the ordeal in verse:

With eyes half open, mind half-shut,

We stagger from the weighing hut,

And climb upon our equine friends

Like divers suffering from the bends; . . .

And yet each year we keep returning

Like Moslems for their Mecca yearning.

Forgotten last year's vale of tears,

The emptiness between the ears . . .

Forgotten last year's bleeding stern,

We ask for more -- we never learn.

Christmas Goose and Mellon won. They won the next year, too, and the year after that.

Mellon is at home at Oak Spring near Upperville, Va. He has spent all morning in a pasture meeting with MacKenzie Miller, his trainer, discussing foals and forage. Now luncheon is almost over.

The conversation turns to Stubbs' 18th-century paintings, those borrowed from the house for the National Gallery's exhibit. Mellon started his collection with Stubbs' "Pumpkin and a Stable Lad." He acquired it in England while studying at Cambridge in 1931. It cost him "a few hundred pounds." It may now be worth half a million. Ten thousand pictures later it is still among his favorites.

"It was through horses," he has said, "that I really got interested in art."

The whipped cream on the shortcake is no thin, store-bought concoction. It comes from the estate, from the Brown Swiss and the Jerseys in Bunny Mellon's dairy. She excuses herself briefly as the butler bends to whisper something in her ear. It seems Mrs. Onassis is calling from Manhattan (the two women were already friends more than 20 years ago, when Bunny Mellon planned the rose garden at the White House), and she will soon be at the hairdresser's; the butler has the number. Mellon and his trainer are talking of Stubbs' accuracy and stillness, of the finer points of Thoroughbreds and changes in the breed.

Can one gauge a horse by eye as one might a picture? Can one tell by merely looking, by judging conformation, if a colt is fast?

"I doubt it," says Mac Miller.

Mellon says: "You can tell if he is slow."

He is as proud, or perhaps prouder, of the noblest of his Rokeby Stables Thoroughbreds -- Fort Marcy, Arts and Letters and especially Mill Reef, the little horse who won both the English Derby and the French Prix de L'Arc de Triomphe -- as he is of his Ce'zannes.

He says: "There are bad horses and good horses, very good and great ones. Now, Mill Reef is a great horse. He ranks with Secretariat, Hyperion, Man o' War."

Mellon is a modest man. Rarely when he speaks of his pictures or his lands does he speak with such emotion.

It is love that fuels his praise.

There are horses in his heaven:

In my first interview, of course,

I'll ask St. Peter for a horse.

He'll lead me down the heavenly sheds

Past miles and miles of Thoroughbreds

And say, "Since you've escaped Old Nick . . .

They're on the house; just take your pick.

Paul Mellon, to a stranger, does not seem a selfless man. His manners may be perfect, but he has always had his way. His endless generosities have required little sacrifice. He is not particularly close to either of his children. Neither of his wives -- both of them consumed by interests of their own -- demanded he dispense with his long-established habits. All of his long life, with almost childlike insistence, he has husbanded his pleasures. To any casual observer he seems a man fulfilled. He seems to have discovered that elusive point of balance between passivity and action, privacy and sharing, selfishness and service. But even his close friends mention his reserve, his chilling self-sufficiency. His seems a solitary peace.

Time and time again, when he expresses his affections, he does so by including, by alluding to the horse.

When Mellon's daughter, Cathy, was still a little girl, he planted a circle of American box near his house in Upperville. The French writer Denis de Rougemont, then a Bollingen fellow, who visited the place in 1943, noted in his journal that Cathy "who is 9, believes in Pegasus with all her heart. They planted for her a great circle . . . where Pegasus will descend one day, if not night. And each morning she goes to look closely at the grass to see if there is a trace of a virgin hoofprint."

Billy Wilbur, 71, of Warrenton, is one of Mellon's oldest friends. The field they share is country life. The two men ride together almost every week. Two years ago, when Wilbur told Paul Mellon that he was getting married, Mellon made an odd request. Would Wilbur mind, Mellon asked, if he named a horse for him? It was not until the next spring, when Mellon named his yearlings, that Wilbur understood.

Billy Wilbur, the horse, is by Blushing Groom out of Admiring.

John Baskett, the Englishman, is another Mellon friend from another field. Baskett is an art dealer, and the two men have been colleagues since Mellon began buying British art in earnest in 1961. Wilbur is a horseman. Baskett is a scholar. When they talk about Paul Mellon he seems two different men.

Baskett speaks of Mellon as a man of letters who often quotes, from memory, chunks of Shakespeare, Chaucer, Keats. The Mellon that he knows is fastidious. "I have seen him use a spotless linen handkerchief to remove a spot of mud from his polished shoes," says Baskett. Wilbur has seen Mellon wading in manure. Baskett speaks in detail of Mellon's connoisseurship; Wilbur is not up to date on the history of art. Baskett used to fox hunt, though he found it "terrifying" (he has a misshapen collarbone to prove it); Wilbur does so still. When Mellon gave up jumping, he gave away his hunting coats; Wilbur got a number, Baskett got one too.

Mellon's tailor is Huntsman's, London. His clothing is impeccable. He likes to quote the poem:

New hat, new boots with glossy tops,

New gloves, the latest thing in crops,

Worn with an air which well expressed

His sense that no one else was dressed.

In 1984, when Diana Vreeland was organizing "Man and the Horse," her costume exhibition for the Metropolitan Museum of Art, she asked Mellon for a loan. He told her, with some pleasure, he was afraid he couldn't help her, that she would have to call his friends.

It is not his wealth alone that separates Paul Mellon from other art collectors -- others have resources, skilled advisers, well-trained eyes. What distinguishes Mellon is the authenticity of his affections.

Literature is an old love. After Yale, class of 1929, and a second degree from Clare College, Cambridge, class of 1931, Mellon married, and then, in 1940, enrolled as a freshman once again at St. John's College in Annapolis. He had decided, he explained, "to make up, to some extent, important gaps in my education." (Five months before Pearl Harbor he left to join the cavalry, later serving in Europe with the Office of Strategic Services. He was mustered out a major.)

"When he started collecting seriously," says Baskett, "English historical pictures were not regarded at all seriously, even by the English, even by the Tate." Though other men were buying French art and Italian, Mellon loved things English, and that included English art. For seven or eight years, before prices zoomed, Mellon -- who eventually accumulated some 1,900 British paintings, 7,000 drawings, 5,000 prints and more than 20,000 rare illustrated books -- had the field to himself. "When he first showed works from his collection," says Baskett, "the English critics wondered what a man of his resources was doing buying English art. Today they charge him with buying up their heritage, but they can't have it both ways."

It is as if Paul Mellon makes his life's decisions only after close consultations with his heart.

Behind his most important choices one can sense the presence of three women he has loved.

His first wife, Mary Conover Mellon (1904-46), "the inspirational initiator, the founding nurturer, of Bollingen Foundation," gave him an example for his highly individual scholarly philanthropies.

His second wife, Rachel Lambert "Bunny" Mellon, is an inveterate collector whose taste runs to things French. It was well after their marriage, as if moved by her example, that he began to forge his two astonishing collections of French and British art.

His third lodestone was his mother, Nora McMullen Mellon (1880-1973).

Andrew W. Mellon, the Pittsburgh financier, was already 45 when he met his future wife while crossing the Atlantic in 1899. She'd been raised at Hertford Castle. (Her father, a brewer of Guinness Stout, was a member in good standing of England's land-owning "Beerocracy.") When she married Andrew Mellon, she was just 19.

She hated sooty Pittsburgh. She had imagined herself, she wrote in explanation, "in the role of the mistress of the manor who lightens the burden of the peasant . . . I would go into my husband's American towns and plan and plant and win the love and affection of his people . . . My first great disillusion came when I learned that his people were not his people at all. 'They are foreigns, Huns and Slavs and such as that . . . ' It made me sick at heart to live in the center of so much I was told to despise . . . The whole community spirit was as cold and hard as the steel it made, and chilled my heart to the core . . .

"I took my baby to Hertfordshire. I wanted to nurse to life in him my own love for the green fields and the open sky. I wanted him away from the gray-smoke and the dust-filled air of my husband's gold and grim estate . . . Nights that I spent in my baby boy's bedroom, nursing these thoughts for the future, my husband, locked in his study, nursed his dollars, millions of dollars, maddening dollars, nursed larger and bigger the cost of priceless happiness."

It was not a happy match. Long before his parents parted, in an acrimonious divorce, Paul must have felt deeply the conflict in their lives. From the time that he was 5 he spent half the year with each.

His father's house, he has written, "was very dark and the halls were very dark and the walls were very dark and outside, Pittsburgh itself was very dark." England was another world. "I remember huge dark trees in rolling parks, herds of small friendly deer, flotillas of white swans . . . soldiers in scarlet and bright metal, drums and bugles, troops of gray horses, laughing ladies in white . . . and always behind them and behind everything the grass was green, green, green . . .

"There seemed to be a tranquility in those days that has never again been found, and a quietness as detatched from life as the memory itself."

To see Paul Mellon now, there among the rolling hills, gazing at the Thoroughbreds grazing in the meadows, one cannot help but feel that he has somehow managed to recapture that old dream.

Not far from his Oak Spring house, beyond the tended garden, Mellon has built his wife a white-walled garden library, designed by Edward Barnes. It now contains perhaps 4,000 volumes, many of them rare, illustrated books on herbs and landscape gardening, vegetables and flowers. Bunny Mellon was still a child when she fell in love with gardens. "I was 12 years old," she says, "when I bought my first garden book with my first allowance." She pulls it from a shelf to display its colored illustrations. A large yellow abstract painting, a canvas by Mark Rothko, hangs there on the wall. The furniture is handmade, oak. She says "the wood is off the place."

"I wouldn't have married Paul," says Bunny Mellon, "if he hadn't been a poet."

Paul Mellon is at home in his quiet house in Washington. He has already bid good morning to Beverly Carter, who keeps track of his pictures. He might have hired an expert, a National Gallery scholar or a specialist in English art, but Carter was a local girl who had just finished high school in Middleburg, when she entered his employ 20 years ago. He calls her "my keeper." It is not clear whether he is using the word in its British sense, as "curator," or in its zoo sense. Brannigan, her 145-pound Irish wolfhound, growls -- with what one hopes is pleasure -- when Paul Mellon arrives.

Over coffee Mellon says: "The thing I envy about Bunny is that from the age of 5 or 6, her whole life has been occupied by horticulture, by one consuming thing. She had a garden when she was 5. That led her into all kinds of other things -- to trees, to landscape gardening. Everything she does in life -- her reading, her architecture, her love of pictures -- is related in one way or another to this one main interest. To me, that is a very lucky thing for a person to have. I don't have that. I am interested in too many things, in horses, in pictures, in the farm. I can easily see -- not that I would have done it -- that a life of only breeding horses would be terribly interesting. My own problem, at least in my own mind, is that I've never been able to go into things deeply enough. You'd be surprised how little I know, really, about English pictures."

Self-portraits by Vuillard, Toulouse-Lautrec and Degas hang behind him on the wall. "I have missed so many things."