Hydrogen peroxide injected into the inflow cannula of isolated ventilated rat lungs produced a dose-dependent vasoconstriction in the range 0.25-10 mM, with maximum response between 2-5 mM. The effects of H2O2 can be influenced by ionophores or specific inhibitors of ionic channels or pumps. A key role is played by sodium ions which govern the subsequent inflow or outflow of calcium, an ion mediating the vasoconstriction. A physiological role for H2O2 generated by NADPH oxidase is postulated.