Oral administration of probenecid augmented urinary clearance of, trichloroacetic acid precipitable 131I-insulin in six men who were injected with tracer dose of 131I-insulin intravenously. The increments were 16 to 423 % above the control value (P<0.05). In other eight men, endogenous insulin in urine and plasma was measured by radioimmunoassay method. Apparently increased responses in urinary insulin excretion and urinary insulin clearance were observed after probenecid administration. The percentage increases of the insulin excretion and the insulin clearance beyond the control values were 2 to 200 (P<0.01) and 7 to 200 (P<0.01), respectively.It was suggested that insulin was reabsorbed at the proximal convoluted tubules and that probenecid affected this reabsorption mechanism.