A multifrequency calorimeter has been designed to measure the amplitude and time regime of the enthalpic fluctuations associated with structural or conformational transitions in biological macromolecular systems. The heat capacity function at constant pressure is directly proportional to the magnitude of the enthalpic fluctuations in a system. Biological macromolecules undergo thermally induced transitions of different kinds. Within the transition region, these systems exhibit relatively large enthalpy fluctuations that give rise to the characteristic peaks observed by conventional differential scanning calorimetry. The multifrequency calorimeter developed in this laboratory has been designed to measure the frequency spectrum of the enthalpy fluctuations, thus allowing us to estimate thermodynamic parameters as well as relaxation times. This information is obtained from the attenuation in the amplitude or phase-angle shift of the response of the system to a periodic temperature oscillation. This instrument has been used to study the gel-liquid crystalline transition of phosphatidylcholine bilayers. The frequency-temperature response surface for large dimyristoyl phosphatidylcholine vesicles has been measured in the frequency range 0.04-1 Hz. The data are consistent with two enthalpic relaxation processes with time constants on the order of 3.8 s and 80 ms at the midpoint of the main gel-liquid crystalline transition.Biological macromolecules are highly dynamic systems, characterized by different types of intramolecular motions covering a wide range of length and time scales (1). These motions can be classified as (i) those associated with spontaneous structural fluctuations within specific equilibrium states and (ii) those arising from transitions between different macromolecular conformational states (2). These fluctuations in macromolecular structure are coupled to enthalpy fluctuations whose overall magnitude is reflected in the heat capacity function. During the last decade, significant advances have been made in the characterization of the spatial distribution and time regime of these fluctuations (1) where Pj(T) is the population of molecules in the ith macromolecular state, Cpj(T) is the heat capacity of that state, and Cpex(T) is the excess heat capacity arising from fluctuations between states (2). In general, the excess heat capacity is maximal within the transition region and tends to zero at temperatures away from this region. The exact functional form of the excess heat capacity function depends on the nature of the transition mechanism for a particular system (8).