The effects of combined physical and psychological stress on cardiovascular and respiratory responses were examined. Thirty-six undergraduate men performed a mental arithmetic task and moderate aerobic exercise, separately and in combination, while physiological measures were recorded continuously using electrocardiography, impedance cardiography, and respiratory gas analysis techniques. Cardiovascular responses during the combination of exercise and mental arithmetic stress were greater than those during either stressor alone, indicating that exercise did not mask the increases in cardiovascular performance evoked by psychological stress. In contrast, respiratory responses to the combined stressor were greater than those during isolated mental arithmetic, but less than those during isolated exercise. Thus the results indicate that physical and psychological stress exert a synergistic impact on cardiac performance, but not necessarily on respiratory performance. The results are consistent with the notion that the cardiovascular response to acute psychological stress exceeds concurrent metabolic demands.