The effects of salinity (10, 17 and 35 ppt) on O(2) consumption, CO(2) release and NH(3) excretion by crabs and oxidative stress parameters and antioxidant defenses of its tissues were reported. An increase in salinity caused a decrease in O(2) consumption and CO(2) release and an increase in ammonia excretion by crabs. Lipid peroxidation, protein carbonyl, H(2)O(2) levels and total antioxidant capacity of the tissues elevated significantly at 35 ppt salinity except in abdominal muscle where H(2)O(2) content was low. Ascorbic acid content of tissues was higher at 17 ppt salinity than at 10 and 35 ppt salinities. With increasing salinity, a gradual decrease in SOD, an increase in catalase, no change in GPx and a decrease followed by an increase in GR activities were recorded for abdominal muscle. While for hepatopancreas, an increase followed by a decrease in SOD and catalase, decrease in GPx and GR activities were noticed with increasing salinity. In the case of gills, a decrease followed by an increase in SOD, a decrease in catalase and GPx and an increase in GR activities were noted when the salinity increased from 10 ppt to 35 ppt. These results suggest that salinity modulation of oxidative stress and antioxidant defenses in Scylla serrata is tissue specific.