Understanding the biological function of the fat retained during pregnancy is important when estimating energy needs during reproduction. Women as well as rats are often considered to gain fat at specific anatomical sites during pregnancy and use this fat as a source of energy during lactation. However, mobilized body fat covers only a minor part of the energy needed by the lactating rat dam. In this paper, fat cell size, lipoprotein lipase activity, as well as triglyceride turnover in parametrial and subcutaneous adipose tissues were studied during the first 2 weeks of gestation and in virginal controls to further explore the metabolic and physiological basis for changes in body fat during reproduction in rats. Pregnancy increased the size of parametrial but not of subcutaneous adipocytes. The lipoprotein lipase activity in subcutaneous adipocytes was not increased by pregnancy. The accumulation in adipose tissue of 14C from orally administered 14C-oleic acid was higher in virginal than in pregnant rats. No effect of pregnancy on the rate of lipid turnover in parametrial or subcutaneous adipocytes was found. The findings are in accordance with the contention that body fat gained during rat pregnancy, to a large extent, is a consequence of a general growth of maternal tissues rather than the result of a stimulating effect of pregnancy on fat accumulation by adipocytes from specific body sites.