Abstract: AimTo study the metabolites of R,S-1-(2-methoxypheyl)-4-［3-(naphthal-yl-oxy)-2-hydroxypropyl］-piperazine, (naftopidil, NAF), a novel antihypertensive drug in rat plasma. Methods The rat plasma samples were analyzed by LC/MS after oral administration of NAF. According to MS relativity of metabolites and parent compound (NAF) and metabolic rule of compound with similar structure, the structure of potential metabolites were postulated. Phase I metabolites were identified by HPLC/MS and by comparison with authentic standards, phase II conjugates were indirectly identified with <>-D-glucuronidase in presence or absence of glucuronidase selective inhibitor D-saccharric acid <>-1,4-Lactone. ResultsPhase I metabolites desmethyl-naftopidil (DMN), (phenyl) hydroxynaftopidil (PHN), (naphthyl)-hydroxy-naftopidil (NHN) were separated and identified in rat plasma by comparison with reference substances, phase II conjugates, NAF and NHN glucuronide conjugates were separated and tentatively identified by hydrolysis with glucuronidase, the aglycones, NAF and NHN , were identified in rat plasma. ConclusionThe major metabolic pathway of NAF in rat plasma should be the hydroxylation of the phenyl or nephthyl moiety of NAF and demethylation of NAF. Therefore, (naphthyl) hydroxyl-metabolite and NAF followed by conjugation with <>-glueuronic acid.