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NO concentration and its isotopomer ratios were measured over a wide area from San Diego to Honolulu in the eastern subtropical North Pacific (ESNP). Waters in the study area had an NO maximum (38.2-50.5 nmol kg) at 600-1000 m depth, which is similar to the profiles obtained previously in other areas in the North Pacific. We separated the seawater into five water masses (two for the surface layer, two for the middle layer, and one for the deep layer) and deduced NO production-consumption mechanisms in each water body by use of NO isotopomer ratios. The results showed that the mechanisms differ slightly among water masses. In the 'coastal' surface layer, NO is produced by nitrification (NHOH oxidation). In the 'open ocean' surface layer, it is produced mainly by nitrifier denitrification and to a lesser extent by nitrification under substrate-limited conditions. In both 'upwelling' and 'open ocean' middle layers it is produced mainly by denitrification and to a lesser extent by nitrifier denitrification. It is also partly reduced. In the deep layer, it is produced predominantly by denitrification with partial reduction. In this way, isotopomers aid elucidation of production-consumption mechanisms of NO in the sea even though the mechanisms cannot always be ascertained. [ABSTRACT FROM AUTHOR]