Donald G. Welsh, 72, former assistant solicitor for the Interior Department and a government lawyer for 36 years, died Wednesday at his Hyattsville home after a heart attack.

A native Washingtonian, Mr. Welsh was a 1922 graduate of Catholic University. He also pursued graduate work at Carnegie Institute of Technology. In 1934 he earned a degree from the old Washington College of Law, now part of American University.

Mr. Welsh began as a lawyer with the old Post Office Department, serving later as a lawyer specializing in chemical industry cases with the U.S. Patent Office.

He served during World War II with the Petroleum Administration for War, and later was chief counsel for the Bureau of Mines. He retired in 1959 from the Interior post.

Mr. Welsh was a member of St. Peter's Lutheran Church in Northeast Washington, whee he served as trustee and president of the Church Council. Active in the Boy Scouts for more than 30 years, he received the Silver Beaver Award for outstanding service in 1956 from the Scouts' National Capital Area Council.

A 32d degree Mason, Mr. Welsh also was a member of the Sons of the American Revolution, the Society of the War of 1812, the Clan Gregory Society and the St. Andrews' Soceity.

Survivors include his wife, Helen, of the home; three sons, the Rev. Donald K. Welsh of El Dorado, Kan., James G. of Staunton, Va., and David M. of Needham, Mass., and eight grandchildren.