Pregnant rats were exposed to a room temperature of 37° C for 7 hr daily during the last two-thirds of gestation. Controls were kept at 23° C. All rats were kept at 23° C during the subsequent lactation. A system of fostering was employed that gave information on lactational performance of mothers, on growth potential of pups, and on the effect of fostering itself. Exposure to heat had no effect on the gestation period or the litter size, but a depression of foetal growth was observed. Lactation was impaired in the mothers exposed to heat. Food intake and body weight were reduced, and water consumption increased during heat exposure; both food and water intakes were reduced in the subsequent lactation. Young taken from mothers exposed to heat and subsequently fostered to control mothers failed to increase their depressed neonatal weights to control levels. The effects of heat exposure during gestation appear, therefore, to elicit a permanent change in the metabolism.