The denaturation of creatine kinase in guanidine solutions has been followed by both CD changes and the increase in rapid reacting SH groups. The rates of the exposure of SH groups are in general agreement with the ultraviolet absorbance and fluorescence changes reported previously whereas changes in the ellipticity of the enzyme molecule can be detected at low guanidine concentrations before significant changes in the exposure of the aromatic residues could be observed. On the other hand, the rates of changes in the mean residue ellipticity at 220 nm are clearly slower than the changes in ultraviolet absorbance, fluorescence and exposure of SH groups. It is suggested that the secondary structure of the external regions of the peptide chains is affected at low guanidine concentrations followed by gross changes in the tertiary structure of the molecule resulting in the exposure of the buried aromatic residues. The destruction of ordered secondary structure of the peptide chain is a slower process than the opening up of the folded tertiary structure of the molecule.