The energy-transfer mechanism in the proton-transfer reaction of formamidine in aqueous solution was investigated by using the chemical reaction molecular dynamics method. On the basis of our previous studies (J. Am. Chem. Sec. 1991, 113, 769; Can. J. Chem. 1992, 70, 377), the reaction was described as the proton-transfer reaction of a supermolecule consisting of formamidine and a reactive water molecule (i.e., the FW supermolecule) surrounded with medium water molecules. It was found that, in relaxation of the reaction energy, after fast damping of the potential energy of the FW supermolecule with a sudden increase in the vibrational kinetic energy of the supermolecule and a slight and gradual increase in the rotational energy, they decrease gradually, accompanied by dissipation of the solute energy into the medium. Some internal vibrations of the FW supermolecule remained unrelaxed within the time duration of the present simulations, i.e., 0.5 ps. These qualitative features were in good agreement with the study of Gertner et al. (J. Am. Chem. Sec. 1991, 113, 74). Characteristically, the change of the interaction energy between the FW supermolecule and the medium water molecules was found to be small during the course of the reaction. This feature is contrasted with the case studied by Gertner et al. Since the present system is not involved in charge redistribution, the reorganization of medium water molecules is not essential for the reaction occurrence. In addition, the fact that there was no recrossing in the transition state region is also attributed to the same reason. Analysis of the work done on the FW supermolecule by medium water molecules has shown that the componential work vertical to the molecular plane contributes to the energy supply into the FW supermolecule although the energy relaxation process itself takes place. The medium water molecules that solvate the FW supermolecule by hydrogen bonding were also found to be the most influential in energy flow, whether they influenced the mechanism positively or negatively. In particular, it was remarkably found that most of the medium water molecules that provide the most and least work for the reaction progress are localized in the regions of those protons which locate far from the reaction site.