Although the tertiary structures of mitochondrial cytochromes c (cyts c) seem to be remarkably similar, there are variations in their amino acid sequences, stability and functional properties. GdnHCl-induced unfolding experiments on engineered yeast and horse cyt c were carried out with the aim to to clarify, at molecular level, some aspects concerning the stability of this class of proteins. The results obtained are discussed in the light of the three-dimensional structures of the two proteins.