The transient time history of the energy distribution function of alpha-particles in the D-T fusion reactors is obtained by numerically solving a Fokker-Planck equation, by taking into consideration the possible alpha-particle loss from the core-plasma volume. On the basis of the results obtained, the total amount of energy transfer to background plasma and the efficiency of the alpha-particle heating, together with the partition of energy transfer to electrons and to ions, are calculated. In addition, a new fact has been found, i. e. the effect of an 'inertial' property of the alpha-particle distribution function on the transient response of the total amount of energy transfer for a temporal perturbation of the parameters of background D-T plasma, which is important in connection with stability and control problems of D-T reactor-core plasmas.