Exposure of corn seedlings to different concentrations of the toxic heavy metal mercury(II) led to significantly altered activities of the enzyme glutathione transferase (GT, EC 2.5.1.18) important in glutathione utilization in this plant. Markedly elevated activities were found in the shoots. In the roots, however, the activities slightly decreased, induction was observed only after 3 days of exposure to 30 μM mercury concentration. The induction of GT activity was generally preceded by a decline. These responses are specific, as indicated by their time course and dose-dependence.