Diabetes is known to result in depression of myocardial function, whereas hearts from insulin-treated diabetic rats exhibit functional characteristics similar to controls. In the present study, we have studied the effect of insulin perfusion on cardiac performance of 3-day and 6-week streptozotocin (STZ) diabetic rats. Three days of diabetes did not result in depressed cardiac performance when the hearts were isolated and perfused in the working heart mode. Increasing the concentration of glucose from 5 to 10 mM in the perfusion fluid did not alter the function in either control or in diabetic rat hearts. However, when regular insulin or glucagon-free insulin (Humulin) (5 mU/mL) was included in the perfusion medium, the ventricular function of hearts from control rats was significantly enhanced, while diabetic myocardial function remained unaffected. When the study was repeated on hearts from 6-week diabetic animals, cardiac function of diabetic rats was significantly depressed as compared with controls. As in the 3-day study, contractility was not affected in either group by increasing glucose concentration in the perfusion medium. Again, inclusion of insulin in the medium enhanced cardiac contractility only in control hearts. These results suggest that diabetes results in a loss of myocardial sensitivity to insulin which seems to occur as early as 3 days after induction of diabetes with STZ. The study also demonstrates that the beneficial effects of in vivo insulin treatment on myocardial alterations induced by diabetes are not due to its direct myocardial effects.