An attempt to quantify the cycling of N in the oceans suggests that losses of combined N greatly exceed gains, a discrepancy that points to the inadequacy of our knowledge of the processes involved. Combined N in particulate matter sedimenting into deep water is partly returned to the euphotic zone in the form of nitrate, the rate of this upward flux of new nitrogen being the main determinant of the upper limit of biomass in any given area. Within the euphotic zone combined N is rapidly recycled by bacteria and animals and in oligotrophic areas the ammonium, urea, and perhaps organic, N that they make available is the main source maintaining phytoplankton growth.