Leslie K. Falk, a retired assistant chief of technical services at the National Library of Medicine, died Monday at Sibley Memorial Hospital following a heart attack.

Mr. Falk was born in Copas, Minn., graduated from Hamline University in St. Paul, Minn., and then was a school teacher in Puerto Rico. He later earned a master's degree in English at the University of Minnesota and a bachelor's degree in library science at the University of Illinois.

He worked for the Minnesota Historical Society until 1934, when he moved to Washington and joined the staff of the old Army Medical Library. This later became the National Library of Medicine and is part of the National Institutes of Health. Mr. Falk remained there until his retirement in 1964.

He was a private consultant on library research until his second retirement about 1977. Among his clients was the Federal Library Committee, for which he wrote a guide to federal library procurement.

Mr. Falk was a member of the American Library Association. He and his wife, Dixon, lived in Falls Church for many years. Since 1968, they had made their home in Alexandria.

In addition to his wife, Mr. Falk is survived by a brother, retired Army Col. Byron A. Falk of Washington, and two sisters, Mrs. Ward Young of Farmington, Minn., and Mrs. R. C. Rose of West Palm Beach, Fla.