Sunlight photolyzes nitrite in seawater: NO2-+HOH+h&ngr; = NO+OH+OH-. We studied nitrite loss and nitric oxide production attributed to this reaction in surface waters of the equatorial Pacific near 170¿W. Net photochemical loss rates of 2-15% per day were derived from two different types of laboratory incubation experiments. The net nitrite loss rate in the surface water of this region is calculated to average 4¿10-13 mol 1-1 s-1 during the day, or ~6¿10-2 mol m-2 y-1. Nitric oxide was detected in situ with a floating gas-seawater equilibrator. NO was always detectable in nitrite-containing seawater during the day but was undetectable at night or in nitrite-free water. Near sunrises and sunsets the estimated NO vapor pressure, pNO(sea) covaried with the ambient UV insolation in air according to log pNO(sea) = a log UVair+b. Best-fit values to the in situ data indicate a?1 with r2?0.9; simple kinetic models rationalize a values of 0, 1/2, or 1. During the day, pNO(sea) averaged ~3.1¿10-8 atm, corresponding to ~4.6¿10-11 M aq. The ambient atmospheric pNO was ~104-fold lower, implying a substantial seawater supersaturation and a sea → air flux. From the stagnant-boundary layer model and our measurements, we estimate ~2¿10-16 mol 1-1 s-1 (~1.3¿108 molecule cm-2 s-1) of NO efflux in daylight, an insignificant NO loss from the sea. The photochemical NO source and the estimated dark-reaction sink are, within the accuracy of the data, in balance. These results provide evidence for the presence of NO, a free radical, in surface seawater. They substantiate that photochemical reactions produce measurable concentrations of reactive intermediates in surface seawater and that these enter into rapid secondary reactions. These processes may reach sufficient intensity to provide significant effects, such as sea→air fluxes.