The effects of high concentrations of nitrogen on Ceratophyllum demersum L. were studied. Nitrogen was added in the form of nitrate or ammonia. Growth and morphology were not affected by nitrate up to a concentration of 105 mg l−1. Ammonia, supplied in low concentration during a short period, stimulated growth; higher concentrations, applied for a prolonged period, were toxic. Increased levels of both nitrate and ammonia in the ambient water enhanced the nitrogen content of the plants. The amino acid concentration and composition, however, remained relatively constant.