Thirty-seven healthy type A men (mean age 42 years) were randomly assigned to either an aerobic exercise training group or to a strength and flexibility training group. Before exercise, subjects underwent comprehensive physiologic and behavioral assessments, including graded exercise treadmill testing with direct measurement of oxygen consumption (VO2) and measurement of cardiovascular (heart rate, systolic and diastolic blood pressure and rate pressure product) and neuroendocrine (epinephrine and norepinephrine) responses to mental arithmetic. The aerobic exercise consisted of walking and jogging at an intensity of greater than or equal to 70% maximal heart rate reserve for 1 hour 3 times/week for 12 consecutive weeks. The strength training consisted of 1 hour of circuit Nautilus training 2 times/week for 12 weeks. At the completion of the exercise program, all subjects underwent repeat testing. For the aerobic group, peak VO2 increased significantly from 33.6 to 38.4 ml/kg/min (p less than 0.001), whereas the strength group only achieved a slight increase from 34.5 to 35.6 ml/kg/min (difference not significant). During the mental arithmetic, the aerobic group experienced a greater reduction in levels of heart rate, diastolic blood pressure and rate pressure product than the strength group (after completing the exercise training programs). The aerobic group also tended to secrete less epinephrine and to show a faster recovery than the strength group after the exercise program. In addition, the aerobic group tended to exhibit less cardiovascular reactivity to mental stress after exercise training. These data suggest that aerobic exercise reduces levels of cardiovascular and sympathoadrenal responses during and after mental stress.