In rats of different ages the furosemide concentration in the kidney was measured at different times after the administration of 0.6 mg/100 g b. wt. furosemide i.p. The accumulation of furosemide was studied on renal cortical slices. The maximal concentration of furosemide in kidney tissue can be reached more rapidly in 55-day-old rats than in 5-day-old rats. There are no differences in medullar and cortical concentrations of furosemide in the kidney. In the renal cortical slices from 55-day-old rats the accumulation of furosemide against the medium is 4 to 6.6 fold in dependence on the initial concentration in the medium. In contrast to experiments with PAH, the accumulation of furosemide is reduced though not abolished in a N(2-)atmosphere. Renal cortical slices from 5-day-old rats cannot accumulate furosemide. The efflux of furosemide and PAH, respectively, from renal cortical slices from 55-day-old rats is not different. The data are discussed in connection with previous results on the age dependence of pharmacokinetics and efficiency of furosemide.