Under spontaneous conditions in man and dog, very little ascorbic acid is excreted in urine. Ascorbic acid clearance (C ascorbic acid) is promptly augmented when plasma ascorbic acid is increased by intravenous injection. No net tubular secretion of ascorbic acid is demonstrable in either man or dog when plasma ascorbic acid is elevated to levels as high as 12 mg/100 ml in man, and 28 mg/100 ml in the dog. Nevertheless, both in men and the Dalmatian dog, when the glomerular filtration rate (GFR) is decreased, excreted ascorbic acid in relation to the amount filtered is exaggerated so that C ascorbic acid:GFR approaches unity. It is possible that secreted ascorbic acid is masked under ordinary circumstances, with a more significant contribution of secreted ascorbic acid to total urinary ascorbic acid becoming apparent under conditions of low GFR. In man, when the plasma ascorbic acid level is raised to above 6 mg/100 ml, C urate:GFR rises from control value of 0.081 +/- 0.020, to 0.116 +/- 0.026. In both mongrel and Dalmatian dogs an effect of ascorbic acid on urate excretion is not conclusively shown. The uricosuric effect of ascorbic acid in man may be due to competition with uric acid for renal tubular reabsorptive transport. The difference in the metabolism of ascorbic acid in the dog as compared to man may help account for the inconsistent effect of ascorbic acid on uric acid excretion in the dog.