Aquatic weed (Spirodela polyrhiza L.) was acclimatized to NaCl solutions of various levels of salinity. A uniform decrease in catalase activity suggested a loss of hydrogen peroxide detoxification ability. On the other hand, an increase in superoxide dismutase, guaiacol peroxidase, and glutathione reductase supported the existence of a biochemical defense mechanism against free radicals generated during exposure to short-term salinity stress. However, lipid peroxidation and total peroxide content increased with increasing salt stress imposition. These results suggest peroxidative degradation of membrane lipids in the root cells exposed to high salinity stress. A higher accumulation of proline at higher concentrations was assessed, implicating proline as avoiding water deficit and stabilizing cell membranes and proteins.