Fingerlings of estuarine fishes, Tilapia mossambica and Lates calcarifer were exposed to sub-lethal concentration of mercury and chromium (2.8 ppm) for a period of 28 days. When these fish were exposed to metals concentration, severe gills alterations were observed. But the alteration was less in fish T. mossambica when compared to that of L. calcarife. The fish L. calcarifer exposed to mercury plus chromium, showed lifting up of the epithelium, swelling, hyperplasia, hypertrophy, proliferation of chloride cells, but in mercury treatment, lamellar fusions, fused secondary lamella and necrosis were observed, whereas in T. mossambica the gills disintegration of epithelial cells, desquamated epithelium, hemorrhaged and exhibited complete damage of epithelial cells of lamellae. The Na+, K+-ATPase activity of both gills and plasma showed significant reduction throughout the experiment period in both fishes. The enzyme activity was more drastic in the case of plasma. The results are discussed in relation to the significance of the above enzyme as non-specific biomarkers against environmental stress.