In this paper, we describe the heat-capacity changes under heating as a process defined by phonon–vibrational exciton interactions. Following Davidov, we describe an α-helix as three noninteracting linear molecular crystals. Further development is achieved by solving the Boltzman's transport equation in the second quantization approximation for the quasi-particles: phonons and vibrational excitons. Heat capacity is defined in terms of the distribution function changes with temperature. We found out that heat absorption is realized through the transfer energy from the molecular vibrations to the internal vibrational degrees of freedom in the physiological temperature range. © 1993 John Wiley & Sons, Inc.