6-Arylamino-5,8-quinolinediones 3 and 7-arylamino-5,8-isoquinolinediones 4 were synthesized as inhibitors of endothelium-dependent vasorelaxation. The quinones inhibited the vasorelaxation of rat aorta with the endothelium. Among them, the quinones 3a, 3b, 3f, 4b, 4d and 4g strongly inhibited the vasorelaxation.