The effects of maternal exercise on pregnant women and fetal well-being are largely unknown. Forty-eight pregnant women between 16 and 39 weeks' gestation were exercised on a bicycle ergometer. We studied the oxygen consumption, blood pressure, maternal and fetal heart rate (FHR) at rest, during and after the exercise. The mean maternal heart rate and blood pressure were increased to 166.1 +/- 12.2/min (mean +/- S.D., n = 48) and 161.1 +/- 20.1/82.7 +/- 15.2 mmHg, respectively, at maximal exercise. The absolute oxygen consumption (1/min) was increased with advancing pregnancy at rest and maximal exercise, but the functional oxygen consumption (ml/kg/min) was not changed during pregnancy. The mean FHR was increased about 4 and 9 bpm in the 2nd and 3rd trimesters, respectively. Abnormal FHR patterns after the exercise were observed in 8 cases (16.7%), mild tachycardia: 6 cases, deceleration: 2 cases. Increasing the maternal heart rate at maximal exercise, increased the frequency of the abnormal FHR pattern. When the maternal heart rate was below 160/min, there was no abnormal FHR pattern. These results suggest that several medical checks should be done not only for the mother but also for her fetus during exercise and the maternal heart rate should not exceed 160/min.