Effect of cold acclimation on fasting-induced adrenocortical and metabolic responses was studied in rats. The animals were acclimated to cold by exposure to 5 degrees C for 4 weeks. The warm control animals were kept at 25 degrees C for the same period. Fasting for 48 h decreased the body weight to the same extent in warm controls (WC) and cold-acclimated rats (CA). Plasma free fatty acids concentration increased by fasting, but the degree of increase did not differ between WC and CA. Plasma glucose level decreased in WC, while it increased in CA by fasting. Plasma corticosterone level was significantly higher in CA than in WC. Fasting increased the level in both groups, and the extent of increase as assessed by 95% confidence interval was significantly greater in CA as compared with that in WC. The present results indicate that fasting-induced glucocorticoid secretion is potentiated by cold acclimation, suggesting a possible role of this hormone in some metabolic changes in fasting.