In rats of different age (5 to 240 days old) the renal excretion of Trishydroxymethylaminomethane (THAM) was studied. In 5 and in 240 days old rats the renal excretion of THAM was slower than in rats of other age groups. Stimulation of diuresis by i.p. injection of mannitol, thiazide or by oral water load resulted in an increase in THAM excretion in 5 and in 240 days old rats. The renal excretion of THAM was also increased by repeated administration of THAM in all age groups, except in new born rats. Possible mechanisms of action are discussed.