Arnold Olsen, 73, a former member of of the U.S. House of Representatives from the western district of Montana, died of heart ailments Oct. 9 at a nursing home in Helena, Mont.

Mr. Olsen, a Democrat, served in the House from 1961 to 1971. He was a member of the Post Office and Civil Service Committee, the Interior Committee and the Public Works Committee. He had particular interest in environmental and labor issues, and he was regarded as a supporter of measures to better the lot of federal employees.

Defeated for reelection in 1970, Mr. Olsen returned to Butte, Mont., where he practiced law. He was elected a state judge in 1975 and continued on the bench until his death. He was stricken while attending a judicial conference last summer.

A native of Butte, Mr. Olsen attended the Montana College of Mineral Science & Technology and received a law degree at Montana State University. He practiced law in Butte until World War II. He served in the Navy in the Pacific.

In 1948, he was elected attorney general of Montana. He served until 1956, when he was an unsuccessful candidate for governor, losing to Hugo Aronson, a Republican. In 1958, he was an unsuccessful candidate for the Montana Supreme Court.

Mr. Olsen's wife, Margaret Olsen, died in 1987.

Survivors include three daughters, Rae Childs and Karen Billings, both of Helena, Mont., and Kristine Graetz of Billings, Mont.; two brothers, Paul Olsen of Billings and Ralph Olsen of Butte, and four grandchildren.