Aquatic organisms have been considered to concentrate metals several times greater than environmental levels. Fishes have been used for many decades to evaluate the pollution status of water and thus considered as excellent biological indicator of heavy metals in aquatic environments. Heavy metals are natural tress components of the aquatic environment, but their levels have increased due to domestic, industrial, mining and agricultural activities. These heavy metals when accumulated in the fish tissues, they damage and weaken the mechanisms concerned leading to physiological, pathological and biochemical changes. The lead is non essential element while cobalt is an essential element for living organisms but its presence in fresh water in higher concentration are toxic to organism's brain, liver, ovary, kidney and gills of the fish. The present study was aimed to investigate the changes due to two heavy metals (lead & cobalt) on the activity of the antioxidant enzyme, Catalase (CAT), Reduced glutathione (GSH), and Lipid peroxidation (LPO) in the brain of Danio rerio during 5, 10, 15 and 20 days of exposure period. For this study adult fishes were exposed to four different concentrations viz., 20, 30, 40 and 50 mg/l of cobalt and 5, 9, 13, and 17 mg/l of lead.