James MacGregor Byrne, 70, a former New York lawyer and retired Foreign Service officer, died of a heart ailment Friday in his Chevy Chase home.

Mr. Byrne entered the Foreign Service in 1941 as a special assistant to the ambassador to Argentina, a post he held for four years. He later served as U.S. consul in Bern, Switzerland, and held posts in Ethiopia and Tunisia.

He had several assignments in Washington, including that of deputy to the director of the Foreign Service senior seminar. He also had been a member of the U.S. mission to NATO before retiring in 1964.

For the next four years he was deputy chairman of the Foreign Students Service Council before retiring a second time in 1969.

Mr. Byrne was a native of Oyster Bay, Long Island, and a 1931 graduate of Harvard University. He earned a law degree at Harvard three years later, then practiced law in New York City until joining the Foreign Service.

Survivors including his wife of 32 years, the former June Beakes of Chevy Chase; a son, James Jr., of Washington; three daughters, Moyra of Austin, Tex., Jill, of Quebec, and Jaird, of Boulder, Colo., and a sister, Phyllis Cox of Cambridge, Mass.