The effect on renal function (creatinine clearance [Ccreat] and inulin clearance [Cinulin]) of changes in chronic dietary protein intake was studied in seven healthy male subjects. Serial 24-hour urine collections were used to determine creatinine excretion (UcreatV) and Ccreat. Subjects were examined after ad libitum (ad lib) food intake and after 2-week periods of high protein diet ([HPD] 1.6 g/kg body weight [BW] per day) and low protein diet ([LPD] 0.5 g/kg BW per day). Inulin clearance (Cinulin) was determined at the end of each 2-week diet period. UcreatV increased from 1,838.8 +/- 97.2 mumol/kg (20.8 +/- 1.1 mg/kg) BW to 2,068.6 +/- 106.1 mumol/kg (23.4 +/- 1.2 mg/kg) BW daily during HPD and decreased significantly to 1,555.9 +/- 167.9 mumol/kg BW per day (17.6 +/- 1.9 mg/kg BW per day) with beginning of LPD. Ccreat rose from 1.54 +/- 0.09 mL/s 1.73 m2 (92.5 +/- 5.5 mL/s.1.73 m2 (104.7 +/- 4.9 mL/min/1.73 m2) during HPD and decreased to 1.23 +/- 0.04 mL/s.1.73 m2 (74.0 +/- 2.2 mL/min/1.73 m2) with initiation of LPD. There was no difference between Cinulin after HPD (1.42 +/- 0.12 mL/s.1.73 m2; 84.9 +/- 7.2 mL/min/1.73 m2) and after LPD (1.36 +/- 0.05 mL/s.1.73 m2; 81.4 +/- 2.9 mL/min/1.73 m2). This study confirms the effect of protein intake on Ccreat and UcreatV, but fails to show an effect of changes in chronic protein intake on glomerular filtration rate (GFR). Ccreat during dietary protein restriction to 0.5 g/kg/d is similar to Cinulin and may be a useful measure of GFR under circumstances where more specific inulin or isotope studies are not available.