The biotransformation of sultopride has been investigated in rat, rabbit, dog and man. In man sultopride was metabolically stable, and about 90% of an oral dose was excreted in urine unchanged and 4% as oxo-sultopride. Rat, rabbit and dog metabolized sultopride more extensively and excreted less than 40% of an oral dose of 14C-sultopride in urine. Four similar metabolites were excreted by the three animal species but the relative portions differed. The major radioactive component in rat urine was O-desmethyl sultopride, whereas oxo-sultopride and O-desmethyl sultopride were the major urinary metabolites in rabbit. Dog formed N-desethyl sultopride and oxo-sultopride as major urinary metabolites. The male rat excreted smaller amounts of unchanged sultopride in urine than did the female rat. The unchanged sultopride excreted in rat urine was increased slightly by repeated administration.