This study investigated the effects of exercise intensity on renal hemodynamics. Three healthy male subjects underwent exercise tests on a bicycle ergometer at 7 different work loads for 15 min. The indicators of exercise intensity employed in this study were the percentage of maximal oxygen uptake (%VO2max) and percentage of ventilatory threshold (%VT). As renal clearance parameters, para-aminohippurate clearance (CPAH) and inulin clearance (CIN) were measured by the continuous infusion technique. Indicators of renal hemodynamics during exercise were the percentage of CPAH, CIN and filtration fraction (FF) at rest. Plasma norepinephrine (NE), plasma epinephrine (E) and plasma renin activity (PRA) were measured. The best regression models between renal clearance parameters and exercise intensity were selected using Akaike's information criterion (AIC). 1) The renal clearance parameters used during exercise were %CPAH, which determined the cubic regression model and %CIN, which determined the quadratic regression model as the exercise intensity increased using AIC. 2) The percentage of maximal oxygen uptake at the onset of decrease in %CPAH and %CIN were 35 and 49%VO2max, respectively, while %CPAH and %CIN began to decrease at 75 and 105%VT, respectively. Intensity of exercise at the onset of increase in %FF was 49%VO2max or 106%VT. 3) The renal clearance parameters during exercise decreased linearly as NE, E and PRA increased. The increase in NE contributed mainly to a change in renal clearances shown by multiple regression analysis. The above results suggest that the relationship among renal plasma flow (RPF), glomerular filtration rate (GFR) and exercise intensity were demonstrated by the cubic regression model and quadratic regression model using AIC, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)