After adrenalectomy in obese mice the rate of weight gain parallels that in lean controls and falls progressively behind that in intact ob/ob mice. Food intake was reduced to normal, but the body temperature of adrenalectomized ob/ob mice was similar to that of intact ob/ob mice and remained below that of control lean mice. After exposure to an ambient temperature of 6 degrees C the body temperature in adrenalectomized ob/ob mice fell at the same rate as in intact ob/ob mice. The increase in tail length, brain weight, spleen weight, and muscle weight after adrenalectomy in ob/ob mice could not be duplicated by reducing the weight gain of ob/ob mice to that of the adrenalectomized mice. Similarly the induction of obesity with gold thioglucose significantly increased the weight of the gastrocnemius muscle and spleen rather than lowering it to levels found in ob/ob mice. Treatment of adrenalectomized ob/ob mice with cortisone acetate, but not with deoxycorticosterone acetate (DOCA), reduced muscle weight, spleen weight, brain weight, and the growth in tail length. Both cortisone and DOCA increased food intake, liver weight, growth rate, and the weight of the adipose tissue in ob/ob mice. These effects are significantly greater than observed in comparably treated lean animals and suggest that glucocorticoids of adrenal origin play an integral role in the phenotypic expression of genetic obesity. These data also indicate that the hypothermia is not a sufficient cause of the obesity in the ob/ob mice, since hypothermia persists in the adrenalectomized mice, but when food intake falls to normal there is no progression of the obesity.