1. On addition of reductant (ascorbate plus NNN'N'-tetramethyl-p-phenylenediamine) to isolated cytochrome c oxidase (ox heart cytochrome aa(3)), in the presence of the inhibitors azide or cyanide, an initial partially reduced species is formed with absorption peaks at 415nm, 445nm and 605nm, which slowly gives rise to the final ;half-reduced' species in whose spectrum the 415nm peak has disappeared and a new absorption is seen at 430-435nm. 2. In the absence of reductant, cyanide forms an initial complex with the enzyme with a spectrum similar to that of the uncombined form, which slowly changes into the ;low-spin' cyanide form with a peak at 432nm. Azide, in absence of reductant, shifts the Soret peak slightly, but the resulting complex, which is probably thermally ;mixed-spin', undergoes no further changes. 3. The Soret-peak shift of oxidized cytochrome a(3) which occurs on reduction of the enzyme in the presence of azide is accompanied by a concurrent blue shift of the ferrous cytochrome a peak from 605nm to 603nm. A partial blue shift of the alpha-peak occurs in the half-reduced sulphide-inhibited enzyme, and a complete blue shift is seen in the analogous complexes with alkyl sulphides [a(2+)a(3) (3+)HSR compounds, where R=CH(3), C(2)H(5) or (CH(3))(2)CH]. 4. Analogous, albeit less readily decipherable, spectroscopic effects with the ligands imidazole and alkyl isocyanides suggest that on reduction of cytochrome a an interaction occurs between the two haem groups involving (i) a high- to low-spin change in cytochrome a(3), and after this, (ii) a change in the molecular environment of the cytochrome a. The latter effect, possibly a decrease in the hydrophobicity of the haem pocket, requires that the ligands on cytochrome a(3) have a bulky and partially hydrophobic character.