As a continuation of our previous studies on thc synthesis and antiplatelet activity of quinolin-2(1 H)-ones with an r-methylidene-;I-hutyrolaetone substituted at 0(8), the O(6)-and N( I)-substituted isomers were synthesized and evaluated for antiplatelet activity against thrombin (Thr).. arachidonic acid (AA)., collagen (Col).. and platelet-activating-factor (PAF)-induced aggregation in washed rahhit platelets. These compounds were synthesized from 6-hydroxyquinolin-2(1 H)-one t'icr alkylation and Re/orniut.skx-type condensation (Schc~mes 1 and 2). All of them were found to inhibit the platelct aggregation perfectly which was induced by AA and Col. 6-Substituted isomers Sb-g exhibited very strong inhibitory activities against AA-and PAF-induced aggregation and are approximately ten times more potent than their X-substituted counterparts. However, the I-substituted (1 la and Ilb) and the 1 -6-disubstituted ( 6 ) counterparts were relatively inactive. Their effects on the Ca2+-dependcnt vasoconstriction induced by high K ' ~ and the phasic and tonic vasoconstrictions induced by norepinephrine (NE) in rat aorta were also evaluated. Except Sg, all of them were found to have significant inhibitory activity on the NE-induced phasic and tonic vasoconstrictions. Compounds 6 and 1 Ib also exhibited strong inhihitory activity on high-K + medium, Ca' ' -induced vasoconstriction.