ATPase melting has been studied by circular dichroism and differential scanning microcalorimetry. Decomposition of the alpha-helix of H+-ATPase (in which about 80% of the peptide groups of the enzyme are involved) following thermal treatment is shown to proceed gradually, beginning with room temperature. Effect of nucleotides upon melting is detected in the range of 20 degrees--40 degrees C. Above 40 degrees C, the pattern of thermal decomposition of the three-dimensional structure of H+-ATPase is independent of the nature of nucleotides present. Highly stable alpha-helical sites have been found in the enzyme molecule. Possible mechanism of formation of such sites is discussed, and the results obtained are compared with data on thermal stability of ATPase from thermophilic bacteria. Structural changes in the molecule following thermal treatment are compared with ATPase activity changes under similar experimental conditions.