This study investigates ammonium (NH4+) uptake kinetics, growth characteristics, and antioxidative response of Lemna minor L. to NH4+ exposure. L. minor was exposed to different NH4+ levels (0.5, 1, 2, 3, and 4 mM) for various contact durations (1, 3, 5, 9, and 14 d). L. minor grew well in 0.5-3.0 mM NH4+, with a relative growth rate of 0.046-0.048 d(-1). The maximum uptake velocity was achieved at 0.066 mg g(-1) FW h(-1), and the growth velocity could be fitted well with the Michaelis-Menten function (R-2 = 0.90507). Plant growth was suppressed, and the chlorophyll and carotenoid contents decreased after "14"-day of exposure on 4 mM NH4+. 4 mM NH4+ induced oxidative stress and heated the antioxidative response in L. minor after "5"-day of exposure. Superoxide dismutase activity increased in contrast to the peroxidase activity decreasing compared with control. The increased malondialdehyde indicated membrane lipid peroxidation because of incomplete antioxidative reactions. (C) 2013 Elsevier B.V. All rights reserved.