Dr. Dorsey Taylor Mahin, 55, a specialist in nuclear medicine who retired in 1972 as chief of medical research for the Department of the Army, died Sept. 29 at the Richmond Memorial Hospital in Richmond after a heart attack.

Dr. Mahin had been chief of nuclear medicine and ultrasonic services at the hospital since 1974, when he moved to Richmond.

A retired Army colonel, he graduated with honors from the Military Academy at West Point in 1945. He served for a short time in the Corps of Engineers and then was stationed in New Mexico, where he was involved with the assembly, testing and delivery of nuclear weapons.

Dr. Mahin, who was born in Lafayette, Ind., left the Army in 1949 to attend medical school. After earning a medical degree in 1955 from the Columbia University College of Physicians and Surgeons, he rejoined the Army as a physician.

In the early 1960s, he was chief of medical outpatient services at the Walter Reed Army Medical Center. From 1964 to 1969, he was director of nuclear medicine in the nuclear medicine division at the hospital's institute of research. He then became chief of medical research for the Army Department.

Following his retirement, Dr. Mahin became a fellow in the nuclear medicine division at the Johns Hopkins University School of Medicine and served for a year as director of nuclear medicine at the Anne Arundel General Hospital in Annapolis before moving to Richmond.

Dr. Mahin was the author and coauthour of numerous research papers in his fields. His professional memberships included the American Medical Association, the American Association for the Advancement of Science, the Society of Nuclear Medicine and the American College of Nuclear Physicans.

Survivors include his wife, Helen, of the home; a daughter, Teal Postula, of Nyack, N.Y.; his mother, Nina C. Mahin, of Frankfort, Ind., and a sister, Nancy Roberts, of Stevens Point, Wis.

The family suggest that expressions of sympathy be in the form of contributions to the American Heart Association.