To clarify renal handling of rhodamine 123, a substrate for P-glycoprotein, in normal and diseased states, in-vivo clearance studies were performed with normal rats and rats with glycerol-induced acute renal failure. For normal rats the excretion ratio of unbound rhodamine 123-to-inulin was 3.25, indicating the presence of the renal tubular secretion of rhodamine 123. Co-administration of cyclosporin, a P-glycoprotein inhibitor, significantly reduced tubular secretion of rhodamine 123. Administration of glycerol induced both an increase in blood urea nitrogen and a reduction in the glomerular filtration rate, confirming the induction of acute renal failure. Total plasma, renal, and tubular secretory clearances of rhodamine 123 were significantly lower for rats with acute renal failure than for control rats. There was no difference between the ATP content of the renal cortex in control rats and those with acute renal failure. In addition to the decrease in renal clearance, a decrease in the biliary clearance of rhodamine 123 was also observed in rats with acute renal failure. These results imply that rhodamine 123 is secreted via P-glycoprotein in renal tubules and that the renal secretory clearance of rhodamine 123 was reduced after acute renal failure, probably because of impairment of P-glycoprotein.