The circular dichroism (CD) spectrum of angiotensin converting enzyme (peptidyl-dipeptide hydrolase, EC 3.4.15.1) in the ultraviolet region was shown to have down negative peaks at 208 and 222 nm, indicating its peptide chain has an alpha-helical structure. The conformational changes of the enzyme during denaturation in guanidine solutions of increasing concentration, for 24 h at 4 degrees C, were associated with the disappearance of the two negative peaks of the CD spectra, less alpha-helical structure to various extents, a decrease in intensity of the intrinsic protein fluorescence, a red shift in the emission maximum at 340 nm and an increase in the band-width of the spectrum delta lambda. Together these findings demonstrate unfolding of the folded peptide chain of angiotensin converting enzyme and consequent exposure of its aromatic amino acid residues during denaturation. The rates of ellipticity (theta 220) changes of the enzyme during denaturation were less than those of the decrease in fluorescence intensity, demonstrating that the rate of degradation of its secondary structure was slower than that of its tertiary structure. Both the rates of inactivation and conformational change of the enzyme increased with increasing guanidine concentrations, within the range of 1.0-3.0 M. The enzyme inactivation had separate fast and slow processes. Both the rates and the extents of inactivation were much faster and larger than those of conformational changes. Compared with other enzymes, therefore, the angiotensin converting enzyme molecule appears to have a stable spatial structure, but its active site conformation is relatively unstable during denaturation.