The renal clearance test was carried out in 6 normal male cats and 12 male cats with chronic renal failure. The average concentrations of creatinine (Cr), urea, sodium (Na), and potassium (K) in the serum of the cats with chronic renal failure were 5.09, 136.7 (mg/100 ml), 143.9 and 3.71 (mEq/l) respectively, and the specific gravity of urine was 1.009. The renal clearances of Cr, urea , Na, and K (ml/min/kg of body weight) were 2.639 ± 0.217, 1.034 ± 0.110, 0.024 ± 0.007 and 0.266 ± 0.028, respectively in normal cats, and were 0.789 ± 0.407, 0.358 ± 0.211, 0.095 ± 0.084 and 0.872 ± 0.204 in cats with chronic renal failure. Clearance of Cr and urea was significantly lower in cats with chronic renal failure than in normal cats, while the values of Na and K were significantly higher in cats with chronic renal failure. The glomerular filtration of Cr and urea and the urinary excretion of these 4 substances were significantly higher in cats with chronic renal failure. The tubular reabsorption rates of Na and K were significantly lower in cats with chronic renal failure compared to those in normal cats, but there was no significant difference in urea and creatinine.