Edwin O. Reischauer, 79, an authority on Far Eastern languages, culture and history who was an immensely successful ambassador to Japan from 1961 to 1966, died yesterday at the Green Hospital of the Scripps Clinic and Research Foundation in La Jolla, Calif.

His physician, Paul J. Pockros, of the Scripps Clinic division of gastroenterology, said Dr. Reischauer died of complications from chronic non-A, non-B hepatitis, a condition he acquired several years ago.

At the time of Dr. Reischauer's appointment as ambassador, he was known to scholars and diplomats here as a Harvard professor who was fluent in Japanese and Chinese and who was arguably the nation's leading authority on Japan. But his career in government service had not been long or extensive. He had never held a diplomatic post.

Yet he became one of the country's most successful and respected diplomats. He traveled to Japan in the aftermath of riots that had prevented a visit by President Eisenhower and in the face of what seemed to be a worsening of America's image in all segments of Japanese society. American and Japanese interests seemed to be colliding on everything from commerce to defense to national philosophy.

Upon reaching Japan, Dr. Reischauer seemed to throw the embassy open to the Japanese people. His fluency in Japanese enabled him to meet, sometimes informally, with students, reporters, labor leaders, intellectuals and women's groups, rather than dealing only with the government and big business. And he also held his own in political and economic negotiations.

To the surprise of some, the ivory-tower professor displayed the gifts of a truly adroit envoy. His greatest talent was the ability to speak with calm and authority to both Americans and Japanese, explaining the mysteries of each to the other.

He seemingly won over an anti-American audience in Osaka with a history lesson. He explained that the Japan of the 1960s was both secure and prosperous because of American sacrifices. He bluntly told Americans they must give up Okinawa, despite its symbolic importance from World War II, because a hostile Japanese public would make American bases there worthless.

He told Japanese leftists that the great world conflict was not between "socialism" and "capitalism," but between free and pluralistic societies and totalitarian systems of government. He genially told Americans that their perception of Japan as a nation coming out of feudalism was just flat wrong. He spoke to Americans about a Japanese democratic tradition that went back to 1880, when local elections were first held, and 1889, when a national parliament was decreed.

Dr. Reischauer, who could be bluff and formidable, was in the end both charming and persuasive. He also avoided several of the great pitfalls of envoys: He did not fall prey to "localitis" -- that is, over-identification with his host country -- and he did not take public positions that went against his government's policies. He also skillfully avoided issues that were not his responsibilities. Along the way, it seems he won the approval of even career diplomats.

The ambassador also was immensely popular with the embassy staff, making a point of meeting and mixing with them. He also opened the ambassador's personal swimming pool to all 2,500 embassy employees.

Edwin Oldfather Reischauer was born in Tokyo to missionary parents. After attending the American School in Tokyo, he came to this country and graduated from Oberlin College with a degree in history in 1931. The following year, he received a master's degree at Harvard. After further studies at the universities of Paris, Peking, Tokyo and Kyoto, he was awarded his doctorate in Far Eastern languages by Harvard in 1938. He then joined the Harvard faculty, where he was to teach off and on until retiring as a university professor of Japanese history in 1981.

During World War II, he was a senior research analyst with the departments of War and State before receiving a commission in Army intelligence. He ended the war with the Legion of Merit and the rank of lieutenant colonel. After the war, he served briefly as chairman of the Japan-Korea secretariat and special assistant to the director of the Office of Far Eastern Affairs.

In 1946, he returned to Harvard, where he taught immensely popular courses, including a legendary course he had taught since 1939 with the great China expert, John K. Fairbank. Their course, universally known on campus as "rice paddies," helped introduce Far Eastern studies to American undergraduate education.

Dr. Reischauer also published a series of books on Japan that gained him recognition not only as perhaps this country's leading academic authority on Japan, but also as an academic who was a gifted writer. His books ranged from language texts and scholarly journal articles to such popular works as "The Japanese Today: Change and Continuity," published in 1988, and "Japan: The Story of a Nation." That book's fourth edition appeared earlier this year.

In October 1960, the prestigious Foreign Affairs magazine published an article he wrote on America's "Broken Dialogue with Japan." It attacked both this country's policies and its lack of understanding of the Japanese culture, people and state.

In 1961, President Kennedy called upon him to take the post of ambassador, a job Dr. Reischauer later said he had not remotely expected to be offered.

He told a Washington Post reporter in 1985 that "I was a specialist in the 9th century, on the travels of a Japanese monk named Ennin in T'ang China. My wife was horrified. But I saw I was being asked to put up or shut up, after I'd been telling everybody what was wrong in Southeast Asia."

His first wife, the former Adrienne Danton, whom he married in 1935, died in 1955. The following year, he married Haru Matsukata, a journalist and granddaughter of Prince Masayoshi Matsukata, former Japanese prime minister. Survivors include his wife, of La Jolla and Belmont, N.Y.; three children by his first marriage, Robert, director of the Congressional Budget Office in Washington, Ann Heinemann of La Jolla and Joan Simon of Larchmont, N.Y.; and nine grandchildren.