Compared with well-nourished controls, progeny of energy-restricted females are smaller and grow less well. The mechanisms behind these effects are frequently studied in rats but interpretation of such studies requires knowledge about the partitioning of dietary energy among the costs of reproduction and maternal needs. Energy partitioning was studied in rats fed ad libitum or 70% of ad libitum intake from 24-34 d before conception until d 14 of lactation and in virginal controls. Body composition was estimated for adult rats on d 1 of gestation, for adult rats and litters on d 1, 9 and 14 of lactation and for virginal controls after 65 d of dietary treatment. Milk production and composition were also studied. Dams in the two groups retained similar amounts of energy during pregnancy. Mobilized body energy corresponded to only 6 and 12% of the energy in milk for ad libitum-fed and restricted rats, respectively. Adult energy-restricted rats were smaller than their ad libitum-fed counterparts and thus needed less energy for maintenance. During lactation, energy-restricted rats used less energy per unit body size than ad libitum-fed rats. Pups of restricted dams were not only smaller, they also utilized milk energy less efficiently than did pups of well-fed dams.