The food intake of sham-operated and adrenalectomized rats was measured in 12-h intervals with animals housed in either alternating periods of 12:12 light-dark or in continuous light. The food intake with alternating light and dark was unaffected by adrenalectomy. The maintenance of a constant infusion of corticosterone in adrenalectomized rats also did not disturb the pattern of food intake. In continuous light, food intake was not significantly different in each 12-h period of the daily cycle. Neither adrenalectomy nor the injection of corticosterone, 240 micrograms/day, altered the average 12-h food intake in rats in constant light. The weight of interscapular brown adipose was smaller in adrenalectomized animals, but the protein content was unaffected. Adrenalectomy significantly increased the specific binding of the purine nucleotide GDP to mitochondria from brown adipose tissue. This specific binding was restored to normal by either corticosterone infusion or injection. We conclude that light is the principal entrainer for the average food intake during 12-h periods and that within the framework of these experiments corticosterone plays an insignificant role in controlling food intake. However, adrenalectomy did significantly increase the purine nucleotide binding to mitochondria from brown adipose tissue.