To investigate the effects of chronic exposure to mercury we studied the red cell antioxidant system in mercury-exposed workers through the evaluation of reduced glutathione, catalase and superoxide dismutase systems. Of these workers, some were being exposed at the time and had presented urinary mercury levels considered safe for occupational exposure for at least 3 months prior to the initiation of this study, and others had been on leave for at least 6 months because of intoxication symptoms. Reduced glutathione levels were lower and catalase activity was higher in the workers which were still being exposed, compared to those on leave and controls. No differences were observed between the workers on leave and controls.