Eight healthy women who were not conditioned athletes, at 35-38 weeks' gestation, performed a submaximal bicycle exercise test three times for 4 minutes each with stepwise increases (73 +/- 27, 114 +/- 29, and 161 +/- 16 W) of the work load. The target heart rate at the end of the last load was 170 beats per minute. The systolic-diastolic ratios (S/Ds) of flow velocity waveforms were measured with pulsed color Doppler ultrasound in the uterine and umbilical arteries immediately after each work load and during recovery. The heart rates at the end of each load (133 +/- 3, 156 +/- 3, and 173 +/- 7 beats per minute) corresponded to an average of 70, 83, and 92% of the calculated maximum heart rate. With the subject in the sitting position on the ergometer before exercise, the S/D in the uterine artery was 1.49 +/- 0.09, increasing gradually and significantly during the exercise to the maximum of 2.04 +/- 0.06 at 1 minute of recovery. Heart rate and S/D in the uterine artery correlated significantly (r = 0.58, P less than .01). Although the mean arterial pressure increased significantly during exercise from 92 +/- 6 to 109 +/- 10 mmHg, the significant decrease of the ratio of the mean arterial blood pressure and S/D suggests that the flow in the main uterine artery may decrease during intense exercise. No changes occurred in the S/D of the umbilical artery flow velocity, but the fetal heart rate increased significantly.