Adult Osborne-Mendel female rats were fed either stan dard laboratory or high-fat diets during the course of pregnancy and lactation. Animals were killed on day 20 of pregnancy and 20 days postpartum after nursing either 0, 4, 8 or 14 pups. High-fat feeding increased total carcass fat, adipose depot weights and fat cell size by two to three fold. Pregnancy also increased carcass fat but the distribution was not uniform; in both dietary groups the parametrial depot was unchanged. The subscapular and retroperitoneal depots increased in pregnant standardfed rats while the subscapular and omental depots increased in pregnant high-fat-fed rats. Increased fat cell size accounted for almost all pregnancyrelated fat deposition. Postpartum depot weights and fat cell size of those standard-fed animals nursing 0 or 4 pups remained near pregnancy values. In contrast, those nursing 8 or 14 pups depleted fat uniformally to onethird the weight at term. High-fat-fed animals lost three times more ab solute fat than standard-fed rats during lactation with disproportionate mobilization from the subscapular and omental depots. Thus, high-fatfeeding, which promoted fat deposition in nonpregnant animals and further exaggerated the fat accretion during pregnancy, did not prevent the ex tensive fat mobilization accompanying normal lactation. It is hypothesized that during lactation hormonally induced metabolic changes in adipose tissue may favor fat mobilization and hence promote weight loss even when a diet which normally induces triglycA©ridestorage is fed. J. Nutr. 120: 600-609, 1980. INDEXING