Portions of diaphragm and heart from genetically diabetic and control mice of three age groups were analyzed for free fatty acid, triglyceride, and carnitine content. Triglyceride levels were increased consistently in both cardiac and skeletal muscle from diabetic animals while the amount of free fatty acids was comparable to that measured in tissue from lean littermates. Free carnitine and total carnitine were decreased in diaphragm and heart from db/db mice throughout the course of the study. While the levels of short-chain carnitine were comparable in tissue from control and diabetic animals, the amount of the long-chain derivative was elevated significantly in both diaphragm and heart in the 18-week-old diabetic mice. The results are discussed with respect to (a) alterations in hepatic carnitine metabolism in this animal model reported previously by this laboratory, and (b) changes in carnitine metabolism which we observed in skeletal muscle from streptozotocin-diabetic animals.