The objective of this study was to investigate the effects of Cd-induced oxidative stress on antioxidative responses of spinach under different K concentrations in hydroponic culture. Plants were grown under controlled conditions at various K (50, 250 and 2010 mu M) and Cd (0 and 20 mu M) supplies with four replicates. Cadmium treatment significantly reduced shoot and root dry matter productions. Potassium treatments decreased Cd concentrations of shoot and root. Shoot lipid peroxidation was increased with Cd treatment. However, sufficient K levels had a reducing effect on lipid peroxidation. The 20 mu M Cd treatment increased both ascorbic acid and SH-compounds. Superoxide dismutase (SOD) and catalase (CAT) activities had similar trends in K and Cd treatments. Compared to control, the 20 mu M Cd treatment with adequate K level caused a distinctive increase on SOD and CAT activities. The ascorbate peroxidase (APX) activity under 20 mu M Cd treatment remarkably increased with increasing K concentration. The glutathione reductase (GR) activity decreased with Cd treatment at all K doses. The 20 mu M Cd treatment slightly increased GR activity with increasing K nutrition. Compared to control, Cd treatment also increased guaiacol peroxidase (GuPX) activity at all K doses. The GuPX activity decreased both in Cd and non-Cd treatments coupled with increasing K nutrition. Results revealed that enhanced K supply played a crucial role in protection of spinach against Cd-induced oxidative stress by decreasing lipid peroxidation and improving antioxidative defense system.