A paucity of data exists concerning the fetal effects of exercise during pregnancy. Seven women who jogged at least 1.5 miles three times per week prior to and during pregnancy were studied during the third trimester of pregnancy (28 to 38 weeks' gestation). Fetal status was evaluated by the nonstress test (NST) before and immediately after maternal jogging of at least 1.5 miles. The seven pregnant women had a total of 30 NSTs--15 prior to and 15 immediately after jogging. All 30 tests were reactive, and there was no significant difference in the monitoring time required to obtain a reactive NST before and after jogging. The prejogging fetal heart rate (FHR) ranged from 140 to 150 bpm, and the postjogging FHR ranged from 180 to 204 in nine of the 15 NSTs. It took a mean of 22 minutes (range, 12 to 30 minutes) for the FHR to return to the prejogging baseline.