Passage of insulin across capillary endothelium was monitored in perfused rat parametrial adipose tissue by the effect of intra-arterially infused insulin on oxidation of [U-14C]glucose to CO2. Glucose oxidation was constant at 34 nmol C.g-1.min-1 for 90 min in tissues perfused with 0 or 50 microU/ml. The rate of oxidation was doubled in 90 min at 100 microU/ml and maximal (4 X control) in 40 min at 200 microU/ml and in 20-30 min at 500 microU/ml. The slow decline in oxidation rate when insulin infusion was stopped suggested that insulin was sequestered in the tissue. Although half-maximal response to insulin occurred in perfused tissues at 100 microU/ml, it occurred at 8 microU/ml in incubated adipocytes and at 30 microU/ml in incubated tissue. In addition, the time required for maximal response to insulin was longer in perfused adipose tissue than in incubated cells and tissues. The data indicate that transfer of insulin from blood to parenchymal cells in perfused tissue was restricted. The minimal amount of insulin needed for a response by adipocytes in perfused tissue was estimated to be less than 1% of that in blood. Our findings are consistent with the concept that insulin is transferred across capillary endothelium by a receptor-mediated process.