Following oral administration of 14C labelled 8-methoxypsoralen (8-MOP) in man the plasma level course, the metabolite-patterns and the elimination of the parent compound and its metabolites have been investigated. Additionally the results discovered have been compared with the data of pharmacokinetics on dog and rat. In man and rat the plasma protein binding of 8-MOP has been determined. Maximal levels of the total radioactivity in the plasma were achieved 2 h after dosing. At this time 8-MOP represents 50% of the radioactivity in the plasma. The plasma protein binding in vitro of 14C 8-MOP valued from 88% to 91% in man, and between 75% and 83% in the rat. Urinary elimination of the total radioactivity as a measure of the extent of absorption varies greatly and depends on the therapeutic formulation being employed. Following the administration of the solution 74% is recovered within 48 h. Faecal elimination of the total radioactivity reached 14% within 3 days. The metabolite-pattern does not show the unchanged 14C 8-MOP. Several polar metabolites occur in the urine among which biochemical conjugates have been recognized. Only polar metabolites are observable in the faeces from which the radioactivity is incompletely extractable. From a comparison of the metabolite profiles, the rat as well as the dog seem to be a useful animal species for experimental investigations with 8-MOP.