Using Megabalanus rosa a sessile filterfeeder, its scope for growth (SFG) by analysis of energy budget were examined and free amino acids composition of whole body tissue were analyzed under the exposure to different concentrations of Hg Cu and Cd. The $96;hr-LC_{50}$ of the barnacle after 96 hr exposure to Hg, Cu, and Cd were 0.220, 0.269 and $1.380;mgL^{-1}$, respectively. Hg and Cu showed stronger toxicity than Cd, while Hg and Cu had similar influence on the survival of the barnacle. SFG of the barnacles exposed to sublethal concentrations of mercury was 18.936 $Jgdrywt.^{-1}hr^{-1}$ in control group and as increase of mercury concentration the SFG remarkably reduced to 0.041 $Jgdrywt.^{-1}hr^{-1};at;0.1;mgL^{-1}$ concentration of Hg. In the case of Cu, the SFG was 29.841 $Jgdrywt.^{-1}hr^{-1}$ in control group and as increase of concentration, the SFG remarkably reduced to -8.304 $Jgdrywt.^{-1}hr^{-1};at;0.1;mgL^{-1}$ concentration. In Cd the SFG was 15.852 $Jgdrywt.^{-1}hr^{-1}$ in control group, and as increasing concentration, the SFG remarkably reduced to -19.490 $Jgdrywt.^{-1}hr^{-1};at;0.1;mgL^{-1}.$ Content of free amino acid (FAA) of whole body tissue of the barnacle was 45084 $mgkg^{-1}$ in control group, but it was reduced remarkably to 28,130, 37,500 and 37,106 $mgkg^{-1}$ at 0.1 $mgL^{-1}$ concentration of Hg and Cu, and 0.4 $mgL^{-1}$of Cd, respectively. Sum of threonine + serine was 1,334 $mgkg^{-1}$ if control but reduced remarkably to 1,223, 849 and 888 $mgkg^{-1}$ at 0.1 $mgL^{-1}$ of Hg and Cu, and 0.4 $mgL^{-1}$ of Cd, respectively.