The gaseous mediators hydrogen sulfide (H2S) and nitric oxide (NO) are synthesized in the body from L-cysteine and L-arginine respectively. Analogous to NO, H2S is enzymatically produced; from cystathionine-γ-lysase (CSE) and cystathione-β-synthase (CBS). Although the physiological significance of H2S is not yet clear its effects are similar to NO in that it exhibits endothelium-dependent vasodilator activity and may play a part in septic and haemorrhagic shock, hypertension, regulation of cardiac contractility and in inflammation. This has prompted the speculation of NO-H2S ‘cross talk’. However, to date, there have been no reports of a chemical or physiological interaction between H2S and NO. Here we show that incubation of H2S gas or the H2S donor, sodium hydrosulfide (NaHS), with a range of NO donors or NO gas in vitro and in vivo leads to the formation of a nitrosothiol molecule as determined by a combination of electron paramagnetic resonance, amperometry and measurement of nitrite [1]. In cultured RAW264.7 cells, cGMP accumulation was not observed unless NO was released with Cu2+. In pharmacological experiments using pre-contracted rat aortic rings, NO donor-mediated vasorelaxation was inhibited by NaHS and restored after the addition of Cu2+. Furthermore, endothelium (NO)-dependent vasodilation induced by acetylcholine and histamine was attenuated by NaHS and L-cysteine but enhanced by the CSE inhibitor DL-propargylglycine. In sharp contrast, endothelium (NO)-independent vasodilation induced by isoprenaline was not inhibited by NaHS or L-cysteine. In the anaesthetised rat intravenous infusion of low concentrations of NaHS significantly increased mean arterial blood pressure whereas higher concentrations exhibited glibenclamide (KATP)-sensitive vasorelaxation [2]. The vasoconstrictor effect of low NaHS concentrations was abolished in animals pretreated with L-NAME whereas NO-mediated vasorelaxation was inhibited by H2S. Finally, using liver homogenates from lipopolysaccharide treated rats we present evidence for the endogenous formation of this nitrosothiol in vivo[1]. These findings provide the first evidence for the formation of a novel ‘nitrosothiol’ generated from H2S and NO which exhibits little or no vasorelaxant activity either in vitro or in vivo. The consequence of the formation of this novel molecule may be to remove endogenous NO to mediate vasoconstriction. We propose that a crucial and unappreciated role for H2S in the vascular system is the regulation of the availability of NO [2,3].