Chromium is a significant mutagen and carcinogen in environment. We compared the effects of tri- and hexavalent chromium on cytotoxicity and oxidative stress in yeast. Cell growth was inhibited by Cr3+ or Cr6+, and Cr6+ significantly increased the lethal rate compared with Cr3+. Both Cr3+ and Cr6+ can enter into the yeast cells. The percent of propidium iodide permeable cells treated with Cr3+ is almost five times as that treated with the same concentration of Cr6+. Levels of TBARS, O-2 (-), and carbonyl protein were significantly increased in both Cr6+- and Cr3+-treated cells in a concentration- and time-dependent manner. Moreover, the accumulation of these stress markers in Cr6+-treated cells was over the Cr3+-treated ones. The decreased GSH level and increased activity of GPx were observed after 300 mu M Cr6+-exposure compared with the untreated control, whereas there was no other change of GSH content in cells treated with Cr3+ even at very high concentration. Exposure to both Cr3+ and Cr6+ resulted in the decrease of activities of SOD and catalase. Furthermore, the effect of Cr6+ is stronger than that of Cr3+. Null mutation sensitivity assay demonstrated that the gsh1 mutant was sensitive to Cr6+ other than Cr3+, the apn1 mutant is more sensitive to Cr6+ than Cr3+, and the rad1 mutant is sensitive to both Cr6+ and Cr3+. Therefore, Cr3+ can be concluded to inhibit cell growth probably due to the damage of plasma membrane integrality in yeast. Although both tri- and hexavalent chromium can induce cytotoxicity and oxidative stress, the action mode of Cr3+ is different from that of Cr6+, and serious membrane damage caused by Cr3+ is not the direct consequence of the increase of lipid peroxidation.