The characteristics of probenecid effect on renal urate excretion in the cat were studied by clearance method and compared with those in the rabbit. In the cat GFR was 3.03±0.09 ml/min·kg, and endogenous plasma urate concentration was 1.12±0.57 μg/ml, which is less than that in the rabbit (3.33±0.46 μg/ml). In the rabbit, FE_ur} was 1.76±0.08 and net urate secretion was observed, while, in the cat FE_ur was 0.70±0.02 and net reabsorption was observed. In the cat FE_ur was dependent on urine flow and independent of plasma urate concentration. In the rabbit FE_ur was suppressed by infusion of probenecid (30 mg/kg -0.6 mg/kg·min) into femoral vein. In the cat the same dose of probenecid increased FE_ur and concomitantly increased urine flow. Thus, an increase in FE_ur by probenecid could be considered to be resulted from a change in urine flow. In the cat infusion of probenecid (2.5 mg/kg·min) into renal artery markedly suppressed FE_PAH, but the effects on FE_ur and urine flow were similar to those when probenecid was infused into femoral vein. These results indicate that in the cat kidney urate filtered through glomerulus is reabsorbed by a probenecid-insensitive mechanism with no evidence for net secretion.