The enthalpy of hydrolysis of the enzyme-catalyzed (heavy meromyosin) conversion of adenosine 5'-triphosphate (ATP) to adenosine 5'-diphosphate (ADP) and inorganic phosphate has been investigated using heat-conduction microcalorimetry. Enthalpies of reaction were measured as a function of ionic strength (0.05-0.66 mol kg-1), pH (6.4-8.8), and temperature (25-37 degrees C) in Tris/HCl buffer. The measured enthalpies were adjusted for the effects of proton ionization and metal ion binding, protonation and interaction with the Tris buffer, and ionic strength effects to obtain a value of delta H0 = -20.5 +/- 0.4 kJ mol-1 at 25 degrees C for the process, ATP4-(aq) + H2O(l) = ADP3-(aq) + HPO2-4(aq) + H+(aq) where aq is aqueous and l is liquid. Heat measurements carried out at different temperatures lead to a value of delta C0p = -237 +/- 30 J mol-1 K-1 for the above process.