Factors that might influence the endogenous creatinine clearance were investigated. The effects of urine volume, diet, and exercise on long term endogenous creatinine clearance were studied on normal individuals using the o-nitrobenzaldehyde method for creatinine. Urine volume variations had no effect on creatinine clearance; protein in-gestion and exercise seemed to increase slightly the clearance values. One hour clearances were similar to 24 hour clearances. Long term creatinine clearances of 39 normal individuals of varying age groups were determined and were found to be similar to reported inulin clearance values of these same age groups.