A physiologically based model for drug reabsorption in renal tubules was applied to sulfanilamide reabsorption kinetics in rats, rabbits and dogs. The reabsorption percent of sulfanilamide decreased with an increase of urine flow rate in the three animal species examined. Furthermore, the reabsorption percent increased with the decrease of the glomerular filtration rate (GFR), even though the urine flow rate normalized by GFR was the same. The power law formula was obtained between AR (l) X Pe and the animal body weight. The power was 0.721, which was less than unity. In contrast, sigma was not dependent on animal body weight. The reabsorption percent of sulfanilamide in man was calculated using AR (l) X Pe and sigma obtained from animal data. The value thus calculated was 50-60%, which was comparable to the reported value (52%) in healthy human subjects. Therefore, the reabsorption percent of sulfanilamide in man was successfully predicted by the extrapolation of animal data.