We evaluated the responses of the fetal-maternal unit to immersion and exercise in the water at 15, 25, and 35 weeks' gestation. As seen by underwater ultrasound, fetuses demonstrated body, limb, and breathing movements. Fetal heart rates (FHRs) were normal, and unchanged from those at rest, during maternal exercise in the water at 60% VO2 maximum. In 21 of 23 cases, post-exercise nonstress tests were reactive within ten minutes. There was no uterine activity seen at either 25 or 35 weeks' gestation. Maternal serum alpha-fetoprotein was unaffected at all gestational ages. Neither maternal temperature nor calculated plasma volume changed during exercise. This general lack of effect contrasts with results from other studies involving similar levels of exercise on land. We speculate that the plasma volume expansion with immersion contributes to the normal FHR responses seen in this study.