There is mounting evidence that the tone, and possibly the structure, of the pulmonary vasculature is regulated by the redox status (GSH/GSSG, NADPH/NADP) of the pulmonary vascular smooth muscle cell. This hypothesis may explain some studies which have examined endogenous mediators and inhibitors of oxidative phosphorylation. An analogous model of redox regulation of cellular function and calcium flux is seen in the pancreatic beta-cell. The importance of redox status in the regulation of enzyme reactivity is well recognized. Sulfhydryl redox status may also be involved in the ability of the carotid body to detect changes in oxygen tension. It is likely that sulfhydryl redox status transduces the effect of changing oxygen tension for many physiologic control systems, including the pulmonary vasculature.