Rats were starved for 6 days to determine whether the "nitrogen sparing" observed during starvation in humans was also present in rats. The urinary nitrogen excretion decreased on the first day, probably due to metabolism of remaining dietary protein. From the second day of starvation to the end of the starvation period, the urinary nitrogen excretion increased progressively. The hepatic glycogen stores were depleted at the end of the first day. The blood glucose concentration remained constant throughout starvation period except for a 15% decrease on the first day. There was increased mobilization of lipid stores, starting on the first day, reflected by an increase in the blood free fatty acids, glycerol and ketone body concentrations. These metabolite concentrations began to increase on the third and fourth day which probably reflected depleted fat stores since no visible body fat was observed by the fourth day. The data indicate that the rat does not spare body protein during starvation, probably because it depletes its glycogen and fat stores rapidly and must then depend on body protein as the major fuel for energy metabolism.