To examine the effects of advancing pregnancy on metabolic responses, 12 women, who were recruited early in pregnancy, were studied during 20 minutes of immersion in 30 degrees C water, followed by 20 minutes of exercise in the water (60% of predicted maximal capacity) and 20 minutes of lateral supine recovery. Each subject completed the trials during the fifteenth, twenty-fifth, and thirty-fifth weeks of pregnancy, as well as a control period 8 to 10 weeks post partum. Resting oxygen uptake increased with advancing pregnancy. Resting oxygen uptake was higher in the water than on land but was not altered by pregnancy. Exercise oxygen uptakes were similar for all trials, but the work load required to elicit the VO2 decreased during the thirty-fifth week of pregnancy. Exercise heart rates followed the same pattern as oxygen uptake. Lactate concentrations declined with advancing pregnancy after exercise. Blood glucose levels were normal for pregnancy but declined slightly during exercise. Blood triglyceride levels were elevated with exercise, with a tendency to increase with advancing pregnancy. Resting plasma cortisol concentrations increased with pregnancy but remained lower during immersion and exercise. These results suggest that pregnancy significantly alters metabolic responses to exercise in the water.