John Hay (Jock) Whitney, 77, who was born into one of America's wealthiest families and went on to notable careers in diplomacy, publishing, film productions and the worlds of art and thoroughbred horse racing, died yesterday at North Shore Hospital in Manhasset, N.Y. He had a heart ailment.

Mr. Whitney was the U.S. ambassador to Great Britain from 1956 to 1961. He assumed the post in the aftermath of the ill-starred British-French-Israeli invasion of Egypt following the nationalization of the Suez Canal. This incident strained relations between Britain and the United States and Mr. Whitney was counted an asset in restoring normal good will.

While he was in Europe, Mr. Whitney began investing in the venerable, ailing and widely admired New York Herald-Tribune. But not even the Whitney millions and lavish infusions of new talent could turn it around. In 1966, in a final effort to save it, it merged with two other newspapers, The New York World-Telegram & Sun and The New York Journal-American. On Aug. 16. 1966, the World-Journal-Tribune ceased publication, a victim of the dynamics of the New York newspaper market rather than a lack of entrepreneurial skill.

It was, perhaps, the only major setback for Mr. Whitney in a business career that began in the traditional and conservative purlieus of Wall Street; went on to the high-rolling, high-flying, and, as it turned out, highly profitable backing of Broadway productions and films; and continued with the provision of venture capital for new enterprises through J. H. Whitney & Co., which the future ambassador founded in 1946.

Mr. Whitney never disclosed what his losses at the newspaper were. But it did not discourage him from publishing. He held on to The Herald-Tribune's European subsidiary, the Paris Herald-Tribune; bought "Parade," a national newspaper supplement; invested in five magazines, including Newsweek, Scientific American and Polo (in his younger days, he was a world-class polo player), and became chairman of Whitney Communications Corp., which, through a subsidiary, has interests in 29 small newspapers in Maryland, Delaware and Florida.

At his death, Mr. Whitney remained chairman of the International Herald Tribune, which is published jointly by Whitney Communications, The Washington Post Co. and The New York Times Co.

In a statement issued at his death, Katharine Graham, the chairman of The Washington Post Co., said: "Jock Whitney was a wonderful friend. . . . Newspeople everywhere will remember the superb job he did in building the great staff at the Tribune, many of whom are the leaders of today's newsrooms all over the country."

Mr. Whitney's communications enterprises came to him after he had added several other fortunes to the $20 million he is said to have inherited from his father, Payne Whitney, and another $20 million he inherited from his mother, Helen Hay Whitney.

Like many of his class, he began with an ordinary job in a Wall Street brokerage firm. Much time went to polo and similar pastimes. In the 1930s, however, he began to make large investments of his own--and he chose as his field the fickle milieu of Broadway. Fortune showered its favors upon him, for among the shows he backed were the long-running classics "Life With Father" and "Streetcar Named Desire."

Then he branched into films, serving as chairman of Selznick International Pictures from 1936 to 1940. The company's productions included "Rebecca" and "Gone With the Wind," both of which were Academy Award winners. "Gone With the Wind" also was one of the great money-makers of all time. It is said that Mr. Whitney paid $50,000 for the movie rights to the novel, which was written by Margaret Mitchell, and turned an after-tax profit of $1.5 million on the film.

With the onset of U.S. involvement in World War II, Mr. Whitney went into public service. His first job in Washington was as director of the motion picture division in the Office of the Coordinator of Inter-American Affairs, which was headed by Nelson A. Rockefeller.

He later was commissioned a captain in the Army Air Forces. He was sent to England and then to the Mediterranean, where he became a colonel on the staff of Gen. Ira C. Eaker, the commander of Allied air forces in that theater. In the summer of 1944, he and four companions were captured by German troops in southern France. They escaped from a train and 18 days later made their way to friendly lines. Mr. Whitney received the Legion of Merit, the Bronze Star, membership in the Order of the British Empire and the rank of chevalier in the French Legion of Honor.

After the war, he divided his capital into four parts. He kept two parts for his personal investments and expenses. A third part went to J. H. Whitney & Co. to generate capital for risky investments. The fourth went to the John Hay Whitney Foundation, the main vehicle for his philanthropies.

The foundation provided scholarships and fellowships to exceptional students handicapped by arbitrary barriers such as race, cultural background or region of residence. It also enabled scholars to go to small, independent schools to teach as visiting professors.

In the public policy sector, the foundation undertook a major study of labor-management relations called "The Causes of Industrial Peace Under Collective Bargaining." Through a series of case studies, it examined ways that labor and management reach agreement. Thus, it emphasized accommodation rather than confrontation. It is regarded as a classic in its field.

As a sportsman, Mr. Whitney enjoyed international ranking in the 1920s and 1930s as a polo player. In 1935 and 1936, he captained the team that won successive U.S. Opens. He later became a major figure in thoroughbred racing. His mother operated the famed Greentree Racing Stable near Lexington, Ky., and she was known as "The First Lady of the American Turf." She won most of the major races of her day and twice saddled winners of the Kentucky Derby. After her death, Mr. Whitney and his sister, Joan Whitney Payson, continued to breed horses at Greentree. They had the horse of the year in 1949 and again in 1953.

Mr. Whitney was a steward of the Jockey Club, a member of the New York State Racing Commission, a president of the Thoroughbred Racing Association and a governor of the Turf and Field Club.

In the world of art, Mr. Whitney had an important collection of modern French and American paintings. He was a trustee of the Museum of Modern Art and a former trustee and vice president of the National Gallery of Art. He was a member of the building committee for the National Gallery's spectacular new East Building.

Although many of his activities, such as polo and racing, were rich men's games, there was one convention that Mr. Whitney shunned. He refused to have his name listed in the "Social Register." He called the publication a "travesty of democracy" with "absurd notions of who is and who isn't socially acceptable."

John Hay Witney was born on Aug. 17, 1904, in Ellsworth, Me., where his parents were vacationing. His family is said to have established itself in the New World in 1635. He was a maternal grandson of former secretary of state John Hay and a paternal grandson of former secretary of the Navy William C. Whitney. As a boy, he is said to have been pudgy and awkward and to have spoken with a stutter. These handicaps disappeared with time.

He was educated at Groton, Yale and Oxford. He then went to work on Wall Street. A lifelong Republican, he held a number of fund-raising posts in the party and advisory posts in the government under President Eisenhower, who also was a personal friend. Eisenhower sent him to London as ambassador.

Mr. Whitney's possessions befitted his birth and station. In recent years, he ran his businesses from a 500-acre estate on Long island that he inherited. He also had an apartment in New York City, a residence, track and stabling for 60 horses at Saratoga, N.Y., breeding farms in Aiken, S.C., and Lexington, Ky., and a 15,000-acre shooting preserve at Thomasville, Ga.

In 1930, he married Mary Elizabeth Altemus. They were divorced in 1940. In 1942, he married Betsey Cushing Roosevelt, former wife of James Roosevelt, the eldest son of President Roosevelt. She survives, as do her two daughters whom Mr. Whitney adopted, Mrs. Ronald Wilford and Kate Whitney, both of New York City.