Many studies have shown that the kidney plays an important role in the metabolism of many proteins and small peptides. To understand insulin handling in the kidney, we examined urinary insulin excretion under several conditions in patients with mutant insulin syndrome (MIS; insulin Wakayama). Urinary excretion of insulin was studied using high-performance liquid chromatography analysis in patients with MIS. In these patients, most of the insulin extracted from a 24-hour urine collection and from urine collected after stimulation of insulin secretion by glucose or glucagon was normal insulin, whereas 90% of serum insulin is structurally abnormal (Leu-A3 insulin). On the other hand, arginine, which is known as an inhibitor of renal tubular reabsorption, increased urinary excretion of Leu-A3 insulin. The ratio of Leu-A3 and normal insulin in urine after arginine was similar to that in serum. A large amount of Leu-A3 insulin is excreted in urine when reabsorption of insulin at renal tubules is inhibited by arginine. These data indicate that normal and Leu-A3 insulin are filtered through the glomerulus with relatively little restriction. Using the fact that basal urine has a high concentration of normal insulin and an extremely low concentration of Leu-A3 insulin, which has less receptor-binding affinity, we speculated some possibilities. One possibility is that both forms of insulin are reabsorbed by the tubular cells, but with different efficiencies. Leu-A3 insulin absorption in more complete, and this suggests differences in the uptake pathways that may account for the differences in response to arginine infusions. Another possibility is that only normal insulin is secreted from tubules into urine which is mediated by receptors. Our results provide new insight into renal metabolism of insulin and showed that MIS is a useful model for studying it.