Sunlight photolyzes nitrite in natural waters, producing free radicals by NO2-+HOH+h&ngr;=NO+OH+OH-. The primary and secondary radicals may react irreversibly with the medium, or regenerate nitrite as proposed for pure water. We measured net nitrite loss to estimate the irreversible component. Compositionally varied nitrite-rich surface seawaters from the Peruvian coastal upwelling were sealed in glass bulbs, sterilized, and exposed to ambient sunlight or held in darkness. Samples showed light-dependent nitrite losses of 5--27% per day, providing the first field evidence for generation of inorganic free radicals in surface seawater and their interaction with the chemical milieu (~90 nmole/l/day). The rate in filtered samples correlated positively with initial chlorophyll a content and negatively with nitrite concentration, indicating that biogenic scavengers and nitrite may compete in a product-determining secondary step. Although nitrate is photolyzable and was present, it did not show photochemical loss.