The effects of a brief period of supine exercise on fetal heart rate (FHR) was studied in 25 healthy, regularly exercising women, 12 in the late second trimester and 13 in the mid third trimester. Although statistically significant increases in maternal mean arterial pressure and pulse occurred, the exercise intensity was mild, with the average percent maximal pulse being 46 +/- 5% and 49 +/- 5% in the second and third trimester groups, respectively. Small increases in FHR were seen in both groups, which were insignificant statistically and physiologically. No patient experienced significant FHR abnormalities as a result of the supine exercise sequence. The findings suggest that pregnant women may continue submaximal supine exercise of short duration into the mid to late third trimester.