To elucidate the mechanisms of the previously observed decrease in adipocyte surface insulin binding in fat diet-induced insulin resistance (Ip et al., J. Lipid Res. 1976; 17:588-99), we performed studies on the distribution of insulin receptors between the cell surface and cell interior, and the extent of internalization and recycling of insulin receptors in adipocytes obtained from fat-fed and glucose-fed rats. Intact cell binding and binding to solubilized cells (total) was decreased by 50% in rats fed fat for 7 days when compared with glucose-fed rats. Incubation of adipocytes with insulin in Tris buffer (100 ng/ml) resulted in a 40-60% decrease in cell surface insulin binding capacity. In two separate experiments, it was found that this insulin-induced receptor loss (%) in fat-fed rat adipocyte preparations was either comparable to that of glucose-fed rats or somewhat decreased. The degradation of the receptors was not affected, as seen by the lack of difference in the chloroquine effect between the two groups. Incubation of fat cells with insulin in tissue culture medium promoted complete reinsertion of receptors into the cell membrane in glucose-fed rat adipocytes, while fat-fed rat preparations demonstrated a significant decrease (37%) in the extent of reinsertion of insulin receptors. Thus, the decrease in cell surface insulin binding and receptor number in fat-fed rat adipocytes is related to an adaptive decrease in the total receptor content coupled with an impairment in the ability to reinsert insulin receptors from the cell interior after insulin-induced internalization.