Seven women at 25 weeks' gestation exercised on land and in the water at 70% maximum oxygen capacity (VO2max) on a bicycle ergometer. Women had significantly lower heart rates and lower systolic blood pressures during immersion exercise. Women had a mean diuresis of 207 ml (6.5 ml/min) during water exercise compared with 98 ml. (2.4 ml/min) during land exercise. Fetal heart rates showed a tendency toward being higher after land exercise compared with water exercise. Six of seven fetuses displayed tachycardia after land exercise compared to one of seven after water exercise. Exercise on land and during immersion at 70% VO2max was well tolerated. However, exercise during immersion offers several physiologic advantages during pregnancy.