Thermal unfolding of stripes prepared from rat uterus has been studied in the presence of nucleotides by differential scanning calorimetry (DSC). Using ADP, ATP and inorganic phosphate analogue orthovanadate, three intermediate states of the ATP hydrolysis cycle were simulated in the uterus stripes. In the main transition of the DSC pattern at least four overlapping endotherms were detected in rigor (AM), in strongly binding (AM . ADP) and weakly binding state (AM . ADP . V-i) of myosin to actin. It was found that nucleotide binding induced a shift of the main melting temperatures (from 60.7 to 61.1 degreesC) and produced changes in the total calorimetric enthalpies (0.45 J/g for rigor, 0.4 J/g for strong binding, and 0.6 J/g for weak binding state). In the Krebs-Ringer bicarbonate buffer containing 100 nM estrogen (Oe) the main transition temperature shifted to 62.4 degreesC and the total enthalpy change was 0.56 J/g. It seems to be an intermediate phase between the strong and weak binding state. The changes of the parameters of the peak functions suggest global rearrangements of the internal structure in myosin heads in the intermediate states.