The present study was performed to explore the antioxidant effect of selenium on damaged brain induced by organic mercury. Male ICR mice were given consecutively 7 injections for 7 days of : (I) sodium selenite 1 mg/kg s.c. alone, (2) methylmercuric chloride 10mg/kg s.c. alone, (3) methylmercuric chloride simultaneously in combination with sodium selenite, and (4) saline alone as control respectively. Based on the above protocol, we monitored various oxyradical scavenging system as well as the finding of electron microscopy. The results from brain tissues were as follows` 1. Selenium inhibited the rate of generation of superoxide radical.