In order to test the safety of hot tub use for persons with heart disease, 15 men with clinically stable coronary artery disease underwent 15 minutes of immersion in a hot tube at 40 degrees C. On another day, they exercised on a cycle ergometer for 15 minutes; target heart rate was determined by standard methods. Tympanic temperature, skin temperature, electrocardiographic findings, blood pressure, plasma catecholamines, subjective comfort, and cardiovascular symptoms were monitored. The peak heart rate was significantly lower during the hot tub session versus the exercise session (85 +/- 14 versus 112 +/- 19 beats/min), as were the systolic (106 +/- 15 versus 170 +/- 21 mm Hg) and diastolic (61 +/- 6 versus 83 +/- 8 mm Hg) blood pressure measurements (P < 0.01). Tympanic temperature increased by a mean of 0.6 +/- 0.3 degrees C during immersion and 0.1 +/- 0.1 degrees C during exercise. No ischemic electrocardiographic changes or clinical complications occurred. Simple ventricular ectopic activity and "just noticeable" chest pain were more frequent during exercise than during immersion. Plasma norepinephrine increased during exercise but not during immersion. These data suggest that hot tub use within these time and temperature constraints should be safe for men with stable heart disease who can follow an exercise regimen at home.