Renal retention of selenium after administration of trimethylselenonium iodide (TMSeI) was studied in vivo in male and female rats during sexual maturation. The selenium level in the kidneys was found to be significantly higher in male rats than in females during the first hour after intravenous (i.v.) administration of TMSeI. The sex-linked difference, manifested already in 21-day old animals, increased markedly during the following four weeks of postnatal life. It was shown that this process may be accelerated by previously increased selenium intake. The sex-linked difference in renal retention of selenium was observed after administration of TMSeI in a wide range of doses: 0.002-10.0 mumol per animal. The size of the applied dose did not affect significantly the kinetic patterns of selenium in the kidneys during the first hour after i.v. injection. The results suggest the existence of a sex-dependent mechanism in the kidneys which may control the rate of excretion of trimethylselenonium ions in urine both under conditions of normal selenium intake and selenium intoxication.