We evaluated the effect of five aerobic exercise machines on uterine activity during the third trimester of pregnancy. Uterine activity correlated with the type of exercise, but not with the level of exertion. At equivalent work loads, the bicycle ergometer led to uterine activity in 50% of sessions, the treadmill in 40%, the rowing ergometer in 10%, recumbent bicycle in 0%, and the upper arm ergometer in 0%. Therefore, the upper body ergometer and the recumbent bicycle appear to be the safest forms of aerobic exercise studied.