Chemical oceanographic data collected on RV Gaveshani cruises 181 and 182 in the Bay of Bengal during March–April, 1987 showed subsurface ammonium maxima in the water column in northern Bay of Bengal. While there is no regular trend either in horizontal or in vertical distribution of ammonium in the central Bay (Lat. 12–14°N), two prominent maxima, one in the euphotic zone near the oxycline (75–150 m) and the other in the oxygen deficient zone (400–600 m) are observed in the vertical distribution of ammonium in the northern Bay of Bengal (Lat. 18–20°N). The subsurface maximum (2·0–3·5 μmol dm −3) in the euphotic zone is attributed to the regeneration of ammonium by zooplankton excretion associated with the stratification of water column during summer (spring). The spatial distribution of recycled nitrogen (ammonium) along the transects showed an increasing trend of regenerated nitrogen from inshore to offshore regions in the northern Bay. The second ammonium maximum (4·0–6·0 μmol dm −3) observed in oxygen deficient region (< 50 μmol dm −3) is assumed to be due to the bacterial reduction of nitrate to ammonium in the denitrification process. Nitrate deficits ( ΔN = 9–12 μmol dm −3) derived from material balance calculations are in close agreement with those reported earlier. Nitrogen transformations in the water column are schematically represented by distinguishing three zones of biological activity on the basis of oxygen concentration levels.