Polymerization of actin was investigated by a microcalorimetric method at 298.15 K. The calorimetric curves showed that polymerization of actin in the presence of Mg2+ at the concentration range 1.00-6.00 mM was an exothermic process. There was only one exothermic peak in the thermal curves under each condition. The average molar enthalpy change of actin polymerization was -0.703 kJ mol(-1). It was still exothermic in the presence of Kt when the concentration was in the range 25.0-200.0 mM. The average molar enthalpy change of polymerization was -3.03 kJ mol(-1). However, the calorimetric curves contained only one exothermic peak when the K+ concentration was lower, in the range 25.0-75.0 mM; there were two peaks, first exothermic and second endothermic, in the calorimetric curves as the K+ concentration was higher, in the range 100.0-200.0 mM. The result not only indicated that calorimetric curves of actin polymerization changed with different concentration of K+ but also showed that there were some differences between K+- and Mg2+-induced polymerization of actin in the presence of 0.1 mM CaCl2 and 0.2 mM ATP. The Delta G(0) of actin polymerization in each condition was also assayed at 298.15 K and Delta S-0 in the corresponding condition were consequently evaluated from the energy-entropy relation at constant temperature and pressure.