In pregnant and non-pregnant female rats at various ages the energy balance was determined to study the age-induced alterations in the mother and conceptus. The animals were anesthetized and the conceptus removed and separated into male, female, and placental tissues. The animals were then killed and the carcasses prepared for energy balance determination. The results obtained showed that the older animals gained less energy in the body and had lower gross food efficiency than the younger animals. The number, energy and weight of the offspring were not affected by the mother's age. Nevertheless, due to its constant growth during life, the rat is not the best model to study age-related processes.