Twelve women early in their pregnancies were recruited to examine thermoregulation during immersion and exercise in the water (30 degrees C). Their responses were compared at 15, 25 and 35 weeks of pregnancy as well as 10-12 weeks post pregnancy to determine whether the responses differ between the gravid and non-gravid woman or were modified during pregnancy. Rectal temperature, mean skin temperature, heat storage, and evaporation were similar during immersion or exercise during the 15th, 25th and 35th weeks of pregnancy. Compared to 10 weeks post partum, pregnancy reduced heat storage, lowered skin temperature and increased evaporative heat loss during immersion and exercise (P less than 0.05). The results suggest that pregnancy causes subtle changes in the mechanism of thermoregulation which tend to increase heat production and improve heat conservation.