Methyl nitrate (CH₃ONO₂) is known to be an important source of reactive nitrogen in the remote marine atmosphere, and it has been speculated that its atmospheric distribution might be explained in part by an ocean source. Measurements made in the N. Atlantic show that dissolved CH₃ONO₂ is present at depths up to 600–800 m. Maximum concentrations are typically found in the upper water column, indicative of an in situ source. Although we show that CH₃ONO₂ can be produced photochemically in seawater in the presence of nitrite, and that this reaction may account for some production in the uppermost waters of the ocean, the maxima below this zone indicate that an additional production mechanism exists.