SUMMARYThe activity of extracted NADH‐NO3−reductase was measured in the marine dinoflagellatesAmphidinium carteriHulburt andCachonina nieiLoeblich. Its activity showed a diel periodicity and wasca.twice as great at midday as at midnight. The enzyme activity was unstable, with anin vitrohalf‐life of 2–3 h. Values of enzyme activity were low or undetectable during lag phase but paralleled the instantaneous growth rate value during log phase. Nitrate reductase activity was not found in the stationary phase of growth, but additions of NO3−resulted in enzyme activity after 24h. WhenA. carteriwas exposed to a series of light intensities for several weeks, the division rate and enzyme activity increased with increasing light intensity up to saturating intensities. In 6 h exposures, enzyme activity decreased with decreasing light intensities below light intensities saturating division rate. Additions of NH4+(0.5–50 μm) toA. cartericultures decreased the amount of extractable enzyme. Thein vitroactivity was not inhibited by similar NH+4co