Rac-metolachlor, a widely used chloracetanilide herbicide, is now being replaced by S-metolachlor in many countries. The enantioselective effects of rac- and S-metolachlor on root growth of maize and rice was studied in hydroponics. Visible morphological changes in root growth were observed after treatment with rac- or S-metolachlor. The main root and lateral roots were shorter in length, and the number of lateral roots was reduced. The half inhibition (IC50,5d) values for root length of rac- and S-metolachlor were 18.86 and 10.61 mu M. respectively, for maize, and 7.33 and 5.35 mu M, respectively, for rice. The root system activity after treatment with rac- or S-metolachlor was lower than that of the control, while the root membrane permeability was higher. The activities of superoxide dismutase, peroxidase, and catalase in the roots were lower after rac- or S-metolachlor treatment compared to those of the control, while the malondialdehyde content was higher. After rice was treated with 3.1 mu M rac- or S-metolachlor, the cell wall separated from the cell membrane, and some destruction of nuclei and organelles was observed. The entire cell was destroyed after treatment with 12.4 mu M rac- or S-metolachlor. The results showed that S-metolachlor has stronger effects than rac-metolachlor on crop roots. (C) 2012 Elsevier B.V. All rights reserved.