The energetics of pregnancy have been assessed in the golden hamster, using continuous whole body indirect calorimetry to determine energy expenditure throughout gestation. Energy intake was unchanged during pregnancy, either on a daily or cumulative basis. The total energy expenditure per animal was, however, significantly higher (14%) in pregnant hamsters than in virgin control animals. The increase in total expenditure was the result of increases in daily energy expenditure over the last one-third of gestation (mean increase 21%), the period during which the energy costs associated with fetal growth are highest. The respiratory quotient (RQ) of the control hamsters was approximately 0.95, but in the pregnant group there was a progressive reduction over the second half of gestation, and by parturition the RQ had fallen to 0.80. The changes in RQ indicate that there is a switch toward the oxidation of fat, away from the oxidation of carbohydrate, in the later stages of pregnancy. Measurements of body lipid suggest that the fall in RQ in the second half of pregnancy is the result of a net utilization of maternal fat reserves; 42% of maternal body lipid was lost during pregnancy, with most of the loss occurring over the final one-third of gestation. Because energy expenditure is increased (relative to virgin controls) without any change in energy intake, it is evident that the efficiency of energy utilization (energy gain per unit of energy intake) is not increased during pregnancy in the golden hamster.