3-(2-Mercaptoethyl)quinazoline-2,4(1 H,3 H)-dione (1; AWD 100-041) is a substance with immunomodulating and immunorestorative activity. After p.o. administration in male Wistar rats at least 7 metabolites are formed and excreted in urine and faeces. The compounds were isolated and identified on the basis of UV and mass spectra. They are S-methylated structures in which sulfoxidation and ring-hydroxylation have been taken place. Four metabolites are also present as sulfate or glucuronide conjugates. The quantity ratio of the phase I to phase II metabolites amounts to 4:1. In the isolated perfused rat liver and rat hepatocyte culture 6 and 5 of the in vivo identified compounds are formed. The sequence of the metabolic pathways could be confirmed by in vitro experiments in which the incubation of synthetically prepared metabolites and the identification of generated biotransformation products were performed. In the lymphocyte and myeloma cell culture solely the disulfide of 1 is formed. After incubation of the S-methyl compound metabolites originate detectable also in vivo. Regarding the main ways of metabolism firstly 1 is attacked by methyltransferases forming the initial metabolite. After that oxidative processes take place leading to the formation of sulfoxides, sulfones as well as ring-hydroxylated compounds. A part of the ring-hydroxylated metabolites are conjugated.