7-(Ethoxycarbonyl)-6,8-dimethyl-2-phenyl-1(2H)-phthalazinone derivatives and several analogues were synthesized and their inhibitory effects on platelet aggregation were evaluated. Structure-activity relationships are discussed. All synthesized compounds showed no appreciable effect on platelet aggregation induced by adenosine diphosphate, but most of them inhibited effectively the arachidonic acid