Previous investigations of the phosphoenolpyruvate carboxykinase (PEPCK) gene have been conducted using rats. In a recent comparative study, we investigated, for the first time, the effects of fasting, refeeding, alloxan-induced diabetes, and insulin treatment on the levels of PEPCK mRNA in mouse liver, kidney, and adipose tissues. As in rats, fasting and diabetes induced, while insulin repressed, hepatic PEPCK mRNA. In contrast, the response of renal PEPCK mRNA to fasting, refeeding, and diabetes in mice differed quantitatively with that in rats: fasting caused a twofold increase in mice and a fourfold increase in rats. Moreover, diabetes, which induces renal PEPCK mRNA indirectly by causing acidosis in rats, was without effect in mice. In adipose tissue, the results of previous studies in both rats and mice have shown that the amount of PEPCK protein and its rate of synthesis are increased by fasting and diabetes and decreased by refeeding and insulin treatment. Thus, it was surprising to find that fasting, refeeding, alloxan-induced diabetes, and insulin treatment had no effect on adipose tissue PEPCK mRNA in either rats or mice.