Nitrite photolysis at natural light intensities and normal nitrite concentrations in seawater produced detectable concentrations of nitric oxide, which was consumed rapidly by a dark chemical reaction in the laboratory. Nitric oxide was also detected in situ in nitrite-rich surface waters of the central equatorial Pacific, where it formed in daylight and disappeared rapidly at sunset. The formation and rapid cycling of nitric oxide implies the presence of other free radicals in seawater, perhaps as intermediates in ongoing autoxidation processes. The central equatorial Pacific is a nitric oxide source to the atmosphere.