ABSTRACT: Increased ammonium (NH₄⁺) concentrations are reported to be an important cause of decline in submersed macrophytes in eutrophic lakes. In this study, a subacute experiment was conducted to examine the effects of various NH₄⁺ concentrations on growth and membrane permeability of 2 submersed macrophytes, Myriophyllum spicatum and Ceratophyllum demersum. Apical shoots of the plants were incubated in modified Hoagland solution with 0.25, 0.5, 1, 2, 4, 8, 16, 32, 64 and 128 mg l^-1 NH₄⁺-N for 4 d. The plants were then collected for examination of cell membrane permeability and the solutions were used to determine the concentrations of NH₄⁺, sodium (Na⁺), potassium (K⁺), magnesium (Mg²⁺) and calcium (Ca²⁺). The results indicate that high NH₄⁺ concentrations had significant adverse effects (e.g. reduced growth and leaf chlorosis) on the macrophytes and increased membrane permeability, leading to net leakage of NH₄⁺ and the cations. Compared to M. spicatum, C. demersum had higher membrane permeability, which might enhance NH₄⁺ transportation across the membrane and thus increase its tolerance to NH₄⁺ stress. Significant increases in the membrane permeability of M. spicatum and C. demersum were observed in the treatments with external NH₄⁺-N concentrations of ≥2 mg l^-1, which fall within the range of NH₄⁺ concentrations in many eutrophic lakes.