The biotransformation of 3'-chloro-2'-(N-methyl-N-[(morpholino-carbonyl)methyl]aminomethyl)benzanilide hydrochloride (fominoben-HCl, PB 89 Cl, Noleptan) was investigated in man. The most substantial metabolites were quantified. The metabolite patterns of fominoben in urine and plasma are compared. Fominoben, is rapidly and extensively metabolized to many metabolites. At the time of the maximum plasma level. 2 h after an oral dose of 160 mg of 14C-PB 89 Cl, only 1.5% of the plasma radioactivity can be measured as the unchanged initial compound. No parent compound can be found in the urine, only in the stool some traces of fominoben can be detected. The biotransformation can be characterized by four main pathways: 1. cleavage reactions, 2. hydroxylations, 3. cyclisations, 4. conjugations. The reactions 1--3 proceed as well simultaneously as successively.