Reaction of cyanide with oxidized cytochrome c oxidase at a low concentration of the ligand and pH > 8 reveals an initial phase, not reported earlier, associated with a small blue shift of the absorption spectrum, which is followed by a conventional red shift of the heme alpha(3+)3. The initial blue shift resembles the spectral changes induced under the same conditions by low concentrations of azide and it is not observed in the presence of 0.3 mM azide. It is suggested that, similarly to NO, cyanide and HN3 cannot only bind to heme alpha 3 but to Cu(2+)B as well, perturbing the spectrum of alpha(3+)3 indirectly. A rapid binding to Cu(2+)B could provide the long-sought intermediate in the cyanide reaction with heme alpha(3+)3, the existence of which is implied by the Michaelis-Menten type kinetics of the latter process.