We compare the output of an 18-box geochemical model of the ocean with measurements to investigate the controls on both the mean values and variation of nitrate δ 15 N and δ 18 O in the ocean interior. The δ 18 O of nitrate is our focus because it has been explored less in previous work. Denitrification raises the δ 15 N and δ 18 O of mean ocean nitrate by equal amounts above their input values for N 2 fixation (for δ 15 N) and nitrification (for δ 18 O), generating parallel gradients in the δ 15 N and δ 18 O of deep ocean nitrate. Partial nitrate assimilation in the photic zone also causes equivalent increases in the δ 15 N and δ 18 O of the residual nitrate that can be transported into the interior. However, the regeneration and nitrification of sinking N can be said to decouple the N and O isotopes of deep ocean nitrate, especially when the sinking N is produced in a low latitude region, where nitrate consumption is effectively complete. The δ 15 N of the regenerated nitrate is equivalent to that originally consumed, whereas the regeneration replaces nitrate previously elevated in δ 18 O due to denitrification or nitrate assimilation with nitrate having the δ 18 O of nitrification. This lowers the δ 18 O of mean ocean nitrate and weakens nitrate δ 18 O gradients in the interior relative to those in δ 15 N. This decoupling is characterized and quantified in the box model, and agreement with data shows its clear importance in the real ocean. At the same time, the model appears to generate overly strong gradients in both δ 18 O and δ 15 N within the ocean interior and a mean ocean nitrate δ 18 O that is higher than measured. This may be due to, in the model, too strong an impact of partial nitrate assimilation in the Southern Ocean on the δ 15 N and δ 18 O of preformed nitrate and/or too little cycling of intermediate-depth nitrate through the low latitude photic zone.