The activity pattern of two lysosomal enzymes, lysozyme and acid phosphatase, and histological changes in digestive gland cells were studied in the green mussel Perna viridis exposed to 25 µgl^-1 of either copper or mercury by sampling on day 0, 7 and 14. In general both the metals induced higher enzyme activity than control mussels. On day 7, the lysozyme activity in mercury exposed mussels was significantly (p<0.01) higher than that of copper, whereas, on day 14, the difference in enzyme activity was not significant (p<0.05) between metals. On the contrary, acid phosphatase activity on day 14 was significantly (p<0.01) higher in copper exposed mussels than mercury. Histopathology of the digestive gland showed tubular dilatation, tubular cell necrosis and destruction of cilia in the digestive diverticula. On day 7, the incidence of tubular changes was significantly (p<0.01) higher in mercury exposed group than copper whereas, on day 14, the incidence of tubular changes was not significant (p<0.05) between metals.