Thomas H. Glenn Jr., 57, a former Rockville business executive who worked for the National Highway Traffic Safety Administration from 1971 to 1977, died of cardiac arrest Friday in the hospital in Dunedin, Fla.

He was founder and president of the old Glenn Engineering, which did government subcontracting, for 10 years before it merged with another company in 1969.

Mr. Glenn spent six years as a physical scientist with NHTSA, where he worked in the field of biomechanics research and development. He helped conduct tests evaluating automobile passenger protection systems such as airbags and seat belts.

A native of Brooklyn, N.Y., he was a long time resident of Derwood, Md., where he lived until retiring to Dunedin in September 1979.

Mr. Glenn served as a Marine Corps pilot in China during World War II and attained the rank of captain. He was a graduate of The Citadel in Charleston, S.C.

He first came to this area in 1950 to work for Reed Research in Washington, then spent the years between 1952 and 1956 in California as a civilian employe of the Navy before returning to Washington.

Survivors include his wife, Jean Bond Blenn, and a daughter, Leslie, both of Dunedin; another daughter, Alison, of Mesa, Ariz.; a son, Coast Guard Lt. (j.g.) David Glenn of Bellview, Wash., and his father, Thomas H. Sr., of Tampa, Fla.