Abstract: A method for distinguishing drug effects on metabolism of endogenously released araehidonic acid(AA)and exogenously added AA was developed by the use of washed rat platelets. The major metabolites of AA in washed rat platelets, measured by HPLC, were 12-hydroxyheptadecatrienoic acid(HHT) and 12-hydroxyeicosatetraenoic acid (12-HETE). Some factors such as concentration of AA added, incubation time and concentration of A23187 which influence the formation of AA metabolites in rat platelets were studied. The formation of HHT and 12-HETE from endogenously released AA in washed rat platelets was dose-dependently reduced by anisodamine (654-2), while the conversion of exogenously added AA to HHT and 12-HETE was not affected by 654-2.The results of the present study indicate that 654-2 decreased the formation of HHT and 12-HETE in washed rat platelets by inhibiting the release of endogenous AA.