Distribution of radioactivity in rats, serum levels in human volunteers and rats and elimination of radioactivity in volunteers, rats, and mice following oral administration of 14C-labelled THIP have been investigated. Peak values of radioactivity in the organs and in serum were seen half an hour after administration, indicating a rapid absorption. Highest concentrations of radioactivity were found in the kidneys, but radioactivity was seen in all investigated tissues including the brain. The radioactivity was mainly excreted with urine (84-93%). Thin-layer chromatography of urine from volunteers, rats, and mice showed that most of the excreted radioactivity corresponds to unchanged THIP. Three metabolites were found in urine from rats each in amounts of 2-7% of total dose given. Two of these metabolites were also found in urine from the volunteers in amounts of 30-35% and greater than 2%, respectively, and in urine from mice in amounts of 21% and 6% of the total dose, respectively. No radioactivity corresponding to unchanged THIP was found in faeces indicating complete absorption of THIP following oral administration. One of the metabolites, the main one in man and mouse, seemed to be glucuronic acid conjugate of THiP, but the chemical structure of the metabolites has not yet been established.