The metabolism of tricyclopinate hydrochloride(TCPN), a new anticholinergic agent, was studied by in situ perfused rat liver preparation. Two metabolites were isolated by HPLC. Metabolite I was identified as an N-demethylation product, while metabolite II was shown to be an arylhydroxylation product. Metabolite I and metabolite II were also isolated from the urine of the rat given TCPN i.g. This result shows that the two metabolites from the perfused rat liver are just the final metabolites produced in vivo. The metabolites in rat liver microsomes were also identified as those just described. These results suggest that the biotransformation reaction of TCPN is mainly catalyzed by the enzyme in rat liver microsomes. The results of experiment on receptor activity indicate that the action potency with mAChR of metabolite I and metabolite II was only 1/20 and 1/50, respectively, of that of TCPN.