The importance of nitrification in the oxygen consumption by deep-sea sediments was investigated by modelling pore water nitrate profiles from 6 northwest Atlantic cores. Total nitrification and denitrification rates were calculated from the thickness of the nitrification layer, the nitrification rate at the sediment surface (N), the coefficient of exponential decrease of the nitrification rate (B), and the first-order rate constant for denitrification. The four unknowns were determined by best fit of the model to the nitrate profiles. The nitrate profile from the furthest offshore station indicated no denitrification, so that only N and B were determined. Nitrification rates ranged from 150 × 10−6 to 3.86 × 10−6 nmole NH4+ cm−2 s at the 1850 m and the 5105 m stations, respectively. As the oxygen consumption by nitrification could account for 35% of the published total oxygen consumption at these stations, nitrification represented a significant aerobic reaction in these deep-sea sediments. Ammonium sources included an upward ammonium flux from deeper anaerobic strata (6%) and aerobic respiration of organic matter (56%) with the remainder presumably supplied by anaerobic respiration within the oxygenated strata (38%). Nitrogen budgets based on sediment traps indicated that nitrification and burial rates agreed within a factor of 2 of sediment trap organic nitrogen fluxes. Also, 70% of the nitrogen that was nitrified or buried was returned as nitrate to the water column.