The effects on cardiac function of feeding a diet high in sucrose to male Wistar rats over an extended period of time (15 months) was examined. This diet produced a diabetic condition which resembled noninsulin dependent diabetes mellitus. Resting hyperglycemia, high circulating insulin and triglyceride levels were observed in these animals. Further, the sucrose fed animals were overweight in comparison to chow fed control animals. Contractile protein Ca2+-ATPase activity was measured as a biochemical estimate of cardiac contractile function. Myosin and actomyosin Ca2+-ATPase activities of isolated myofibrillar fractions from hearts of experimental animals were depressed in comparison to chow fed control rats. Myosin K+-EDTA activity was also altered. The results demonstrate for the first time a defect in contractile protein Ca2+-ATPase activity in rat hearts using a model of noninsulin dependent diabetes mellitus. As the animals were euthyroid, thyroid hormone alterations in these animals were unlikely to influence the results. The results also demonstrate that insulin could not be a direct factor associated with cardiac pathology in diabetes. Instead, cardiac dysfunction may be associated with other, as yet undefined, metabolic abnormalities which accompany the diabetic state.