1. Swiss albino mice and Sprague-Dawley derived rats, 6-8 weeks in age were compared in the fed condition and after a fast of 24 and 48 hr. 2. Plasma glucose was significantly higher in mice than rats in the fed state but this difference disappeared after the 48 hr fast. 3. In the fed condition, plasma FFA and glycerol levels were significantly higher in mice than rats yet did not change during fasting. With starvation, there was a progressive increase in plasma FFA in the rat but no change in plasma glycerol. 4. The content of triacylglycerols in the plasma and liver was comparable in mice and rats in the fed state. Plasma triacylglycerols declined to a similar extent in both species during a fast of 48 hr. In mice, hepatic triacylglycerols increased markedly during fasting but declined in rats. 5. Heart triacylglycerols were higher in mice than rats in the fed state and increased still further during a fast of 48 hr during which time little change was observed in the rat. Myocardial glycogen was comparable in mice and rats in the fed state. With fasting, cardiac glycogen content decreased in the mouse but remained unchanged in the rat. 6. Distinct differences in certain of the parameters of carbohydrate and lipid metabolism were found with mice and rats in the fed and fasted condition.