The purpose of this study was to determine the effects of a six month aerobic exercise training regimen on cardiovascular responsivity to mental arithmetic in healthy middle-aged men and women. Subjects were randomly assigned to a moderate intensity exercise intervention or to an assessment-only control group. Before and after the intervention subjects' heart rates and blood pressures were measured doing a mental arithmetic task (N = 83). Other physiological and psychosocial measures included the Type A structured interview and a maximal exercise treadmill test. Validated adherence to the exercise regimen exceeded 75% and there were significant increases in aerobic capacity in those subjects receiving exercise training. Exercise did not significantly reduce cardiovascular responsivity to the stress task. Type A behavior did not interact with reactivity across exercisers or controls nor was it significantly correlated with adherence. The results are discussed with respect to factors that have been previously reported to potentially influence the exercise/reactivity relationship.