To establish normal values for glomerular filtration rate and effective renal plasma flow for age and sex, the records of 141 healthy renal transplant donors were reviewed. These patients had no history of renal or systemic disease, had normal physical examinations, and had normal laboratory studies in addition to having studies of the clearances of insulin (Cin) and para-amino hippurate (Cpah) preoperatively. Regression analysis of the clearance data revealed no significant sex differences in renal function. A linear age-related decline in renal function begins in the 20's and decreases by 4 ml/min per decade for Cin and by 35 ml/min per decade for Cpah through the 60's, our oldest patients studied; therefore, the absolute lower limits of normal (lower fifth percentile) for Cin and Cpah were calculated as a function of age. Residual function was constant with average postoperative clearances 70% of preoperative values regardless of age.