8-Chloro-11-(4-methyl-1-piperazinyl)-5H-dibenzo-(b,e)(1,4)-diazepine (clozapine, 1) is metabolized in humans by exchange of the aromatic halogen for a hydroxy- or a methylthio-group (compounds 2 and 3). Further metabolites are the N-demethyl derivatives of 2 and 3, the compounds 4 and 5. In addition a clozapine metabolite with the structure 6 with an oxidized piperazine ring was found. The presence of a metabolite with an oxidized sulfur atom is suggested. Possible ways for the formation of these metabolites are discussed.