1. Increased ammonium concentrations and decreased light availability in a water column have been reported to adversely affect submersed vegetation in eutrophic waters worldwide. 2. We studied the chronic effects of moderate NH(4)(+) enrichment (NH(4)-N: 0.16-0.25 mg L(-1)) on the growth and carbon and nitrogen metabolism of three macrophytes (Ceratophyllum demersum, Myriophyllum spicatum and Vallisneria natans) under contrasting light availability in a 2-month experiment. 3. The NH(4)(+) enrichment greatly increased the contents of free amino acids and nitrogen in the shoot/leaf of the macrophytes. This indicates that NH(4)(+) was the dominant N source for the macrophytes. 4. Soluble carbohydrate contents remained relatively stable in the shoot/leaf of the macrophytes irrespective of the treatments. Under ambient light, the starch contents in the shoot/leaf of C. demersum and M. spicatum increased with NH(4)(+) enrichment, whereas V. natans did not exhibit any change. The starch contents decreased in C. demersum, increased in M. spicatum and remained unchanged in V. natans after the combined treatment of NH(4)(+) enrichment and reduced light. 5. The NH(4)(+) enrichment did not affect the growth of the three macrophytes under the ambient light. However, it did suppress the growth of C. demersum and M. spicatum under the reduced light. The results indicate that a moderate NH(4)(+) enrichment was not directly toxic to the macrophytes although it might change their viability in eutrophic lakes in terms of the carbon and nitrogen metabolism.