Apparent blood flow rate in jejunum, kidney, liver or skeletal muscle was directly measured by the hydrogen gas clearance method in 7-week-old rats anesthetized with urethane at four dose levels (0.5, 0.8, 1.0 and 1.5 g/kg, i.p.). The apparent blood flow in each organ or tissue was reduced with the urethane dose. Jejunal blood flow was most sensitive to the dose-related decrease. Assuming that the organ (or tissue) to blood partition coefficient of hydrogen gas is unity and extrapolating the flow data to zero urethane dose for each organ, the flow rate values were almost comparable with the literature data in conscious rats. The applicability of the hydrogen gas clearance method to examine the effect of age on organ or tissue blood flow in rats was also discussed. Organs with relatively high sensitivity to aging were the stomach, jejunum and kidney.