Physiological effects of NH₄⁺-N concentrations (0.5, 1, 2, 4, 8, 16 mg/L) on Hydrilla verticillata cultured in media were investigated at laboratory conditions. The biomass, chlorophyll contents, soluble sugar contents, protein contents, as well as the activities of superoxide dismutase (SOD), peroxidase (POD) and (GS) of Hydrilla verticillata were analyzed. The results showed that low NH₄⁺-N concentration (0.5, 1, 2 mg/L) could simulate the growth of H. verticillata cultured in media while a NH₄⁺-N concentration of above 4 mg/L would deter the growth of H. verticillata markedly. The plants died within three to four weeks when the concentration of NH₄⁺-N in culture solution reached 16mg/L. The chlorophyll contents and the soluble sugar contents of H. verticillata went up with increase of NH₄⁺-N concentration in low NH₄⁺-N concentration experiments (0.5, 1, 2 mg/L NH₄⁺-N). However, when NH₄⁺-N concentration was above 4 mg/L the contents of chlorophyll and soluble sugar in the 24^th d, 32^nd d and 40^th d sampling decreased noticeably compared to the control experiment. In low NH₄⁺-N concentration (0.5, 1, 2 mg/L) experiments the protein contents of H. verticillata decreased initially and increased later; in control experiments the protein content went up continuously; in high NH₄⁺-N concentration (8, 16 mg/L) experiments the protein content decreased remarkably. Overall, the activities of GS, POD and SOD showed no difference in their changing trends; these enzymatic activities increased noticeably under the condition of high concentration of NH₄⁺-N and showed a pattern of initial increasing and subsequent decreasing with activities peaking in the sixteenth or the twenty-fourth day. The results indicate that the increased NH-4-N concentrations were a stress to H. verticillata under eutrophic condition. The increased NH₄⁺-N concentrations could affect the physiological function of H. verticillata, by restraining its growth, and even resulting in its death. Although H. verticillata could resist the changes in NH₄⁺-N concentrations to some extent, its tolerance gradually weakened with time.