Concentrations of dissolved inorganic nitrogen compounds above the pycnocline in the Oslofjord are very low in the summer, with turnover times of the inorganic N pools of no more than a few hours. To investigate the possibility that continued phytoplankton growth in the summer depends on ammonium excretion by microzooplankton, rates of NH 4 + regeneration and assimilation were measured by a 15N isotope dilution method. Daytime regeneration rates at 0–2 m depth were 0–28% of the calculated assimilation rates at ambient NH 4 + concentrations. Regeneration was faster during a dinoflagellate bloom in August than in mixed diatom-dinoflagellate blooms in June and September. Most of the NH 4 + appeared to be produced by juvenile copepods, rotifers, tintinnids, and heterotrophic dinoflagellates in the size fraction 45–200 μm.