To characterize maternal thermal balance during pregnancy, the authors recorded the thermal response to moderate weight bearing exercise stress in four aerobically conditioned pregnant women. Studies were performed in a climate-controlled environment in each trimester of pregnancy and postpartum after modifying the exercise protocol for changes in maternal exercise condition. Core, vaginal, and mean skin temperatures were recorded at rest and during maximum exercise. Mean resting skin temperature increased during pregnancy; mean resting core and vaginal temperatures did not change. Core temperatures did not exceed 39C during exercise. Heat storage (heat content/kg) was not increased as a result of exercise with advancing pregnancy. According to these findings, thermal balance can be maintained with advancing gestation when exercise prescriptions are appropriately modified for conditioned women.