Pregnancy is considered diabetogenic. Although exercise has been advocated to assist in metabolic control of the nonpregnant diabetic individual, there is a paucity of data about the metabolic effects of exercise during pregnancy. To examine whether moderate exertion may be beneficial in the maintenance of maternal carbohydrate homeostasis, glucose and lactate kinetics were measured in the third trimester in five pregnant nondiabetic women (gestational age, 34.2 +/- 0.1 weeks [mean +/- SE]) by infusion of 45 microg x kg(-1) x min(-1) [6,6-2H2]glucose and 70 microg x kg(-1) x min(-1) [U-13C]lactate tracers. Subjects were observed at rest for determination of baseline steady-state kinetics over a 30-minute period, and then they exercised for 30 minutes at 60% maximum oxygen consumption (VO2max) and were evaluated for 30 minutes postexercise. Glucose and lactate kinetics and lactate oxidation were measured throughout the exercise protocol. This study was repeated postpartum in all individuals at least 6 weeks after delivery. Compared with the steady-state preinfusion period, plasma glucose concentration was not elevated during exercise in either group, nor was plasma lactate concentration significantly different in either group. Glucose kinetics did not change during exercise, but lactate kinetics increased in both groups. V02 and percent of lactate C contribution to CO2, an indication of lactate oxidation, increased proportionally in both groups during exercise. Metabolic perturbations, as measured by glucose and lactate kinetics, do not appear to be different during the third trimester of pregnancy during a relatively short bout of exercise compared with the nonpregnant state.