FK409, a novel vasodilator with anti-platelet aggregation activity, has been isolated from the acid-treated fermentation broth of Streptomyces griseosporeus No. 16917, which was cultured on a medium containing NaNO3 for 4 days. FK409 was purified from the culture-filtrate by extraction with ethyl acetate after adjusting the pH to 3.0 with HC1, followed by silica gel chromatography. The molecular formula of this compound was determined to be C8H13N3O4. In vitro, FK409 showed a potent relaxation activity on noradrenaline induced contraction of rat aorta. In addition to the vasodilating activity, this compound also showed potent anti-aggregation activities towards rabbit platelets. In vivo, intravenously administered FK409 resulted in marked blood pressure lowering in rats.