We investigated the extent to which the altered weight gain of rat pups suckled in litters of varying sizes (4, 10, and 16 pups/litter) is attributable to differences in milk nutrient intake. Milk intake was estimated from the rate of dilution over 2 days of a dose of 3H2O administered at 4, 8, and 14 days of age after correction for the water and milk carbon deposited in body tissues over the measurement period. Protein and energy intakes were estimated from the volume of milk consumed by individual pups and the composition of milk from each dam. Significant effects of litter size on milk fat and protein concentration were observed. Weight gain was highly correlated with energy intake in pups suckled in litters of 4 and 10 but not 16. These findings were attributed to a higher energy expenditure of pups less than 10 days old suckled in litters of 16; specifically these pups had higher maintenance energy needs than pups suckled in litters of 4 and 10 and a higher energy cost of tissue synthesis. The latter was ascribed to an ability of immature pups to maximize the efficiency of protein utilization, thereby blunting the deleterious effects of a reduced nutrient intake on protein deposition.