The consumption of garlic is inversely correlated with the progression of cardiovascular disease, although the responsible mechanisms remain unclear. Here we show that human RBCs convert garlic-derived organic polysulfides into hydrogen sulfide (H(2)S), an endogenous cardioprotective vascular cell signaling molecule. This H(2)S production, measured in real time by a novel polarographic H(2)S sensor, is supported by glucose-maintained cytosolic glutathione levels and is to a large extent reliant on reduced thiols in or on the RBC membrane. H(2)S production from organic polysulfides is facilitated by allyl substituents and by increasing numbers of tethering sulfur atoms. Allyl-substituted polysulfides undergo nucleophilic substitution at the alpha carbon of the allyl substituent, thereby forming a hydropolysulfide (RS(n)H), a key intermediate during the formation of H(2)S. Organic polysulfides (R-S(n)-R'; n > 2) also undergo nucleophilic substitution at a sulfur atom, yielding RS(n)H and H(2)S. Intact aorta rings, under physiologically relevant oxygen levels, also metabolize garlic-derived organic polysulfides to liberate H(2)S. The vasoactivity of garlic compounds is synchronous with H(2)S production, and their potency to mediate relaxation increases with H(2)S yield, strongly supporting our hypothesis that H(2)S mediates the vasoactivity of garlic. Our results also suggest that the capacity to produce H(2)S can be used to standardize garlic dietary supplements.