Institutionalized severely and profoundly mentally retarded adults participated in a 7-month program of rigorous aerobic-type exercises. The effects of the treatment on the physical fitness, intelligence, and behavior of subjects were assessed. Fifty men and women were matched in pairs based on IQ, CA, and sex and assigned randomly to an experimental (E) or control (C) group. Those in the E group met 3 hours per day, 5 days per week and received a treatment that consisted of an exercise program that included jogging, running, dance-aerobics, and circuit training. The C group continued their normal institutional training programs. The treatment produced significant improvement in the cardiovascular efficiency of subjects; however, no changes in intelligence or adaptive behavior were obtained. Although standardized tests reflected little improvement in psychological or behavioral variables due to treatment, subjective reports suggest that exercise training may serve as a more practical habilitation program for severely and profoundly mentally retarded individuals than those typically employed in institutional settings.