The stability of the structure of troponin C (calcium-binding components of troponin from rabbit skeletal muscle) has been studied by the scanning microcalorimetry method. It has been shows that: 1. In the presence of divalent ions the protein structure is represented by two practically independent cooperative blocks, one of which contains Ca2+-specific binding sites, and the other (Ca2+, Mg2+)-binding sites. 2. The stability of the cooperative block containing Ca2+-specific binding sites depends only on the concentration of Ca2+ and in its absence the melting temperature of the block decreases to 58 degrees C at neutral pH and low ionic strength. 3. The stability of the cooperative block containing (Ca2+, Mg2+)-binding sites depends on the concentration of Ca2+ or Mg2+. In their absence the stability of the block is so low that its structure is already disrupted at 25 degrees C. The conformational transition observed by different methods when divalent ions are removed is nothing else than the breaking down of the structure of this cooperative block.