The gaseous mediators hydrogen sulphide (H2S) and nitric oxide ((NO)-N-.) are synthesised in the body from L-cysteine and L-arginine, respectively. In the cardiovascular system, (NO)-N-. is an important regulator of vascular tone and its over- or under-production has been linked to a variety of diseases. The physiological significance of H2S is not yet clear but, like (NO)-N-., it exhibits vasodilator activity and may play a part in septic and haemorrhagic shock, hypertension, regulation of cardiac contractility, and in inflammation. To date. there have been no reports of a chemical interaction between H2S and (NO)-N-.. Here we show that incubation of the HAS donor, sodium hydrosulphide, with a range of (NO)-N-. donors and (NO)-N-. gas in vitro leads to the formation of a nitrosothiol molecule as determined by a combination of techniques; electron paramagnetic resonance, amperometry, and measurement of nitrite. We further show that this nitrosothiol did not induce cGMP accumulation in cultured RAW264.7 cells unless (NO)-N-. was released with Cu2+. Finally, using liver homogenates froth LPS treated rats we present evidence for the endogenous formation of this nitrosothiol. These findings provide the first evidence for the formation of a novel nitrosothiol generated by reaction between H2S and (NO)-N-.. We propose that generation of this nitrosothiol in the body may regulate the physiological effects of both (NO)-N-. and H2S. (c) 2006 Elsevier Inc. All rights reserved.