Flash photolysis of thermally stable, photolabile 'caged' precursors permits rapid and precise changes of ligand concentration at their site of action. This approach was used to determine the concentration-dependence and time course of NO-mediated relaxation of aortic smooth muscle, by use of two photolabile NO donors, trichloronitrosylruthenium (Ru(NO)Cl3) and dipotassium pentachloronitrosylruthenate (K2Ru(NO)Cl5). At concentrations up to 500 microM, both compounds were non-toxic before photolysis, and produced non-toxic by-products on photolysis. Photolytic release of NO produced relaxations of intact and endothelium-denuded aortic rings precontracted with noradrenaline (0.1-0.5 microM), with an EC50 for NO-mediated relaxations of 10.5 nM and 13 nM, respectively. NO-mediated relaxations were reversibly blocked by 1 microM oxyhaemoglobin. The time course of NO-mediated relaxation comprised a delay of 3-7 s, followed by a sigmoidal decline in tension with peak rates that were strongly dependent on NO concentration.