Various xanthones as well as quercetin have been shown to exhibit antiplatelet activity. A series of anthraquinones analogues structurally related to xanthones and a series of quercetin-related compounds were tested for their antiplatelet effects. Emodin, frangulin B, kaempferol tetraacetate, quercetin-3-O-galactoside octaacetate, rhamnazin triacetate, and rhamnetin tetraacetate were found to be potent antiplatelet agents, and 1,8-dihydroxy-6-methoxy-3-methylanthraquinone 8-O-rhamnosyl-(1-->2)-glucoside, rhamnustrioside undecaacetate, rutin decaacetate, and quercetin-3-O-(6-O-alpha-L-arabinopyranosyl)-beta-D-galactopyranos ide decaacetate showed significant antiplatelet effects. Quercetin showed vasorelaxing action in rat thoracic aorta.