In the present aquarium study, Oreochromis mossambicus Peters were exposed to two different concentrations of hydrogen sulfide (H2S) (4.9 and 6.6 mg/l), and the changes in lipid peroxidation (LP) products and antioxidants in test fishes were determined in time intervals of 12, 24, 48, 72, and 96 hours. The results showed that with respect to the H2S concentration and duration of exposure, alterations were observed in the concentration of LP products and antioxidants in the various organs of the test fishes. Malondialdehyde (MDA) content increased in the liver, gill, kidney, and brain on exposure to H2S up to 48 hours, and then the MDA content showed steady value up to 98 hours experimental period. Brain and kidney of fishes showed the maximum increase in concentration of reduced glutathione (GSH) on H2S treatment. The gradual decrease in concentration of GSH in the tissues of H2 S-exposed fishes after 48 to 96 hours compared with the control shows the loss of adaptive mechanisms and the oxidation of GSH to glutathione disulphide (GSSG). Slight increase in the activity of GSH-S-transferase and decrease in activity of GSH peroxidase demonstrated the incapability of the vital organs in neutralizing the peroxides generated in the oxidative stress condition.