The recently identified hormone leptin (ob protein) secreted by white adipose tissue is widely thought to provide a feedback signal limiting fat storage by decreasing food intake. By artificially rearing leptin-treated and control littermates fed identical amounts of milk, however, we show here that lean suckling-age rats treated with recombinant murine leptin can reduce fat storage solely by increasing energy expenditure. Continuous measurements of core temperature and metabolic rate show that this increase is not uniform throughout the day but is especially prominent in the morning when rat pups usually conserve energy by entering a torpor-like state. Leptin's alleviation of hypometabolic, torpor-like states is thus not restricted to cases of impaired hormone production but seems instead to be a normal biological function independent of its effects on food intake.