How the ductus arteriosus (DA) closes at birth remains unclear. Inhibition of O2-sensitive K+ channels may initiate the closure but the sensor mechanism is unknown. We hypothesized that changes in endogenous H2O2 could act as this sensor. Using chemiluminescence measurements with luminol (50 [mu]M) or lucigenin (5 [mu]M) we showed significantly higher levels of reactive O2 species in normoxic, compared to hypoxic DA. This increase in chemiluminescence was completely reversed by catalase (1200 U ml-1). Prolonged normoxia caused a significant decrease in K+ current density and depolarization of membrane potential in single fetal DA smooth muscle cells. Removal of endogenous H2O2 with intracellular catalase (200 U ml-1) increased normoxic whole-cell K+ currents (IK) and hyperpolarized membrane potential while intracellular H2O2 (100 nM) and extracellular t-butyl H2O2 (100 [mu]M) decreased IK and depolarized membrane potential. More rapid metabolism of O2- with superoxide dismutase (100 U ml-1) had no significant effect on normoxic K+ currents. N-Mercaptopropionylglycine (NMPG), duroquinone and dithiothreitol all dilated normoxic-constricted DA rings, while the oxidizing agent 5,5'-dithiobis-(2-nitrobenzoic acid) constricted hypoxia-dilated rings. NMPG also increased IK. We conclude that increased H2O2 levels, associated with a cytosolic redox shift at birth, signal K+ channel inhibition and DA constriction.