Changes in renal function, Na+-K+-ATPase activity and PAH transport system in kidney cortex were studied in rats treated with cadmium. Subcutaneous injections of CdCl2 (2 mg Cd/kg.day) for 16 days induced a marked polyuria and a hyposthenuria. These changes were accompanied by increase in urinary protein, glucose, urea, calcium, phosphate, chloride and potassium excretions. The change in urine flow was proportional to the change in total osmotic solute excretion. Creatinine excretion and TcH2O remained unchanged. Na+ excretion was not increased, but the Na+-K+-ATPase of renal cortex was significantly inhibited. PAH uptake by renal cortical slices was markedly attenuated in Cd-treated rats. The Vmax for active PAH influx was drastically reduced, but the Km was not changed. The passive influx and efflux of PAH across the basolateral membrane and the renal tissue oxygen consumption were not apparently altered in Cd-treated animals. These results indicate that 1) the nature of Cd-induced polyuria and hyposthenuria is an osmotic diuresis induced by proximal tubular rejection of various substances, and 2) the mechanism of impaired renal PAH excretion in Cd-treated animals is a loss of organic anion carriers in proximal tubular basolateral membranes.