Retired Army Col. James Thomas Avery Jr., 66, who began his military career in the field artillery in the days when it used horses and who ended it as a specialist in nuclear warfare, died of cardiac arrest Friday at Mount Vernon Hospital in Alexandria.

Col. Avery, who was born in New York City and grew up in Richmond, Va., graduated from the Virginia Military Institute in 1935. During World War II, he served with a horse artillery unit at Fort Myer and later with the 66th Infantry Division in Europe.

Subsequent assignments included the Command and General Staff School and the Army War College. In the late 1950s, he was assigned to the Atomic Energy Commission in Los Alamos, N.M., in connection with nuclear testing programs.

He then served in the Office of the Joint Chiefs of Staff in Washington, working on nuclear deployment problems. He commanded a nuclear weapons center in Germany and then went to the Institute of Special Studies at the Combat Development Command at Fort Belvoir. He retired in 1970.

Col. Avery, who lived in Alexandria, held the Legion of Merit. He was active in the VMI Alumni Association.

Survivors include his wife, the former Hester Vail Kraemer, of Alexandria; three sons, Richard J. and James Thomas III, both of Boston, and Robert Lee II, of Denver; his mother, Daisy May Lester Avery of Richmond, and seven grandchildren.

The family suggests that expressions of sympathy be in the form of contributions to the VMI Alumni Fund, Class of 1935, Box 932, Lexington, Va., 24450.