The isolated perfused rat kidney was used to identify factors responsible for the renal elimination of prednisolone (Pn). Pn was recirculated at initial concentrations varying from 100 to 1000 ng/ml for 90 min. Perfusate and urine samples were assayed for Pn and prednisone by HPLC. Protein binding of Pn was measured by using 3H-Pn and equilibrium dialysis at 37 degrees C. There were no significant differences in perfusate flow, glomerular filtration rate, urine flow, or sodium excretion between control and steroid experiments. Partial metabolism of Pn to prednisone occurred in all studies. The total kidney clearance (CIT) of Pn ranged from 0.39 to 1.24 ml/min/100 g of rat body weight with approximately half of the Pn dose unaccountable for as either Pn or prednisone. The apparent percentage of the Pn dose excreted unchanged in the urine ranged from 1.9 to 6.4% and was not related to Pn dose. The apparent urinary clearances of Pn and its metabolite, prednisone, normalized for inulin clearance (fractional excretion) were variable with means of 0.068 and 0.095, respectively. The fractional excretions of Pn and prednisone were related to the fraction of filtered water excreted but not to perfusate concentration. Thus, the extent of urinary clearance of these corticosteroids is related to glomerular filtration and passive tubular reabsorption. The perfused rat kidney reflects the urinary and renal metabolic clearance of Pn without the complication of dose-dependent disposition.