The objective of this experiment was to study the effects of metalaxyl enantiomers on the activity of roots and antioxidative enzymes in tobacco seedlings. Water culture experiment was conducted to analyze the effects of different concentrations of metalaxyl enantiomers (30 and 10 mg L-1 ) on root activity and leaf superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities and malondialdehyde (MDA) content of tobacco seedlings. The results showed that metalaxyl significantly inhibited root activity and significantly improved leaf SOD, POD, and CAT activities and MDA content. A better physiological response in tobacco seedlings was observed at 30 mg L-1 than at 10 mg L-1 metalaxyl. The stereoselectivity for different enantiomers had no obvious effect on root activity and the leaf POD activity, but it affected significantly the SOD and CAT activities and MDA content. The SOD activity was promoted more by R-enantiomer than by S-enantiomer at 30 mg L-1 metalaxyl, and the same effect was observed on CAT activity from the beginning to the end of the stress period. The MDA content under the stress by R-enantiomer was higher than that under the stress by S-enantiomer at 10 mg L-1 metalaxyl.