It is now established that S-nitroso-albumin (SNO-albumin) circulates at low nanomolar concentrations under physiological conditions, but concentrations may increase to micromolar levels during disease states (e.g cirrhosis or endotoxemia). This study tested the hypothesis that the high concentrations of SNO-albumin observed in some diseases modulate vascular function, and that it acts as a stable reservoir of NO, releasing this molecule when the concentrations of low molecular weight thiols are increased. SNO-albumin was infused into rats to increase the plasma concentration from <50nmoles/l to ~4 µmoles/l. This caused a 29 ± 6% drop in blood pressure, 20 ± 4% decrease in aortic blood flow, and a 25 ± 14% reduction of renal blood flow within 10 minutes. These observations were in striking contrast to those of an infused arterial vasodilator (hydralazine), which increased aortic blood flow, and suggested that SNO-albumin acts primarily as a venodilator in vivo. This was confirmed by the observations that glyceryl trinitrate (a venodilator) led to similar hemodynamic changes and that the hemodynamic effects of SNO-albumin are reversed by infusion of colloid. Infusion of N-acetylcysteine into animals with artificially elevated plasma SNO-albumin concentrations led to the rapid decomposition of SNO-albumin in vivo, and reproduced the hemodynamic effects of SNO-albumin infusion,. These data demonstrate that SNO-albumin acts primarily as a venodilator in vivo, and represents a stable reservoir of NO which can release NO when the concentrations of low molecular weight thiols are elevated.