Hydrogen sulphide has recently been of considerable interest in adult health and disease, with H2S identified as an important gaseous regulatory molecule with many biological and physiological roles, including synaptic modulation, neuroprotection and smooth muscle relaxation. H2S is endogenously produced in amounts capable of causing vasodilatation, thus controlling blood pressure. Despite compelling adult data, almost nothing is known about the role of H2S in the transitional circulation of the neonate. Its contribution to vasodilatation may be crucial for regulation and dysfunction of vascular tone in the neonate. A Vindoline recent piglet study suggests that H2S may be important in at least the transitional cerebral circulation. These data, combined with our observations on NO and CO, led us to hypothesize that H2S would contribute to the excessive vasodilatation observed in preterm neonates in the initial extrauterine period. Specifically, that H2S production would be greater in those infants at greatest risk of microvascular dysfunction �C very preterm male neonates �C and that levels would correlate with microvascular blood flow. One of the major challenges in translating preclinical animal studies to humans is determining a robust, non-invasive method to measure disturbances in H2S signaling. Due to the short half-life and volatile nature of the gas, we pursued an indirect metabolic measure. The metabolism of H2S can be divided into three distinct pathways: oxidation to sulphate, clearance by exhalation and reactions with metalloproteins and disulphide containing proteins. Oxidation to sulphate and subsequent excretion by the kidneys represents the major metabolic and excretory pathway, with urinary sulphate levels representing around 50% of an exogenous dose of H2S administered orally, subcutaneously, intraperitoneally or intravenously. Sulphate is not a suitable analytical target as production from other sources of sulphur swamp the contribution of H2S.Urinary thiosulphate, an intermediate of the breakdown of H2S to sulphate is routinely used as a marker of exposure to high H2S levels in cases of industrial or environmental exposure and thus represents a better analytical target when the issue is total body Gomisin-G turnover of H2S.