Insulin binding to isolated cardiac myocytes from normal and streptozotocin-induced diabetic rats was investigated. We found that at high affinity sites, the maximum numbers of insulin binding sites per cell are 33 000 and 22 000 for normal and diabetic myocytes, respectively with no discernible difference in receptor affinity. However, since the yield of myocytes from the diabetic heart was only 1/3 of the normal heart, it is suspected that the insulin function in the diabetic heart may be significantly lower than that in the normal heart. Chloroquine was found to markedly decrease insulin degradation with concomitant increase in net insulin uptake by isolated myocytes. This suggests that insulin degradation may take place within lysosomes after insulin is internalized. To determine whether internalization of insulin in myocytes is an energy dependent process, insulin binding and subsequent degradation were assessed in cells depleted of ATP by treatment with various metabolic inhibitors (2,4-dinitrophenol, NaF and iodoacetic acid). Depletion of the cellular ATP level resulted in a decrease in both insulin uptake and degradation. In diabetic myocytes, the general relationship between cellular ATP level and insulin uptake and degradation was similar to that found in normal myocytes. However, in diabetic myocytes, the cellular ATP level and insulin uptake were lower, but insulin degradation was greater than in normal myocytes. Insulin uptake by normal and ATP depleted cells at 4°C (16h) was lower than at 37°C (1h), while the ATP level was almost the same at both temperatures. This suggests that the internalization of insulin is a temperature as well as an ATP dependent process. © 1986 Academic Press Inc. (London) Limited.