The present study investigated the mechanism by which the disposition of insulin is altered in streptozotocin (STZ)-treated diabetic rats as compared with 48-hr-fasted normal (control) rats. It was shown by an indocyanine green infusion method that the hepatic plasma flow rate (QH) in diabetic rats (1.64 ml/min/g liver) is significantly higher than that in control rats (0.982 ml/min/g liver). The portal injection technique revealed that the unidirectional clearance (CLon), which represents the binding of A14-125I-insulin to surface receptors in the liver, is significantly elevated in diabetic rats, suggesting an increase in the surface receptor number (RT), i.e., up-regulation in the liver. In both control and diabetic rats, the total-body clearance (CLtot) and steady-state volume of distribution (Vdss) of labeled insulin decreased significantly with a simultaneous injection of unlabeled insulin (8 U/kg), confirming that the disposition of insulin is affected largely by specific, saturable receptor-mediated processes. The CLtot and Vdss increased significantly in diabetic rats, while nonspecific portions of these parameters were not changed. From the increases in CLtot (80%) and QH (67%) in diabetic rats, a pharmacokinetic analysis has revealed a 40% increase in the hepatic intrinsic clearance (CLint,sp) of A14-125I-insulin via a specific mechanism in diabetic rats. In conclusion, we have provided in vivo evidence for a small increase in CLint,sp of insulin in STZ-diabetic rats compared with control rats, which may be caused by an increase in the surface receptor number in the livers of diabetic rats.