The present studies have examined the diurnal rhythm of corticosterone in genetically obese mice and in mice made obese by treatment with gold thioglucose. The values of corticosterone were significantly higher in the ob/ob mice than in lean mice at each of the four time points measured. The diurnal pattern for corticosterone in ob/ob mice, however, was similar to that in lean mice at both 6 and 12 weeks of age and had a nadir at 0700 h and a zenith at 1800 h. In mice with gold thioglucose-induced obesity, on the other hand, the corticosterone values were the same in the obese and lean control mice 6 weeks after treatment with gold thioglucose. By 12 weeks after gold thioglucose treatment, the obese animals had higher corticosterone levels in the morning than lean animals, but the peak values at 1800 h were not different. Restriction of body weight gain by allowing obese and lean mice food for only 4 h daily altered the pattern of corticosterone, but the values in the ob/ob mice remained higher than those in the lean mice. The corticosterone values of db/db mice housed at 24 C were significantly higher at 0800 and 1800 h than those in lean mice, and the diurnal rhythm was present, but smaller, in the db/db mice. In db/db and lean mice housed at an ambient temperature of 33 C, the corticosterone concentrations increased to the same values, and the diurnal rhythm was blunted in both. At the higher ambient temperature, the db/db mice gained weight more slowly, whereas the lean mice showed no weight gain.