Light-absorption spectra and afferent chemoreceptor discharge were simultaneously recorded on superfused rat carotid bodies (CBs) under the influence of cytochrome a3-Cu-B ligands (O-2, CN-, CO) in order to identify the primary mitochondrial cytochrome c oxidase (CCO) oxygen sensor. Spectra could be described on the basis of weighted light-absorption spectra of cytochrome b(558) of the NAD(P)H oxidase and mitochondrial cytochromes b and c, CCO, cytochrome a3, and an unusual cytochrome a peaking at 592 nm. Discharge signals were deconvoluted into phasic and tonic activity for comparing different CB responses. The spectral weight of cytochrome a(592) decreased significantly starting at high PO2 (100 mm Hg) and low sodium cyanide (CN-, 10 muM) accompanied by increasing phasic peak discharge. Combined CO-hypoxia or CO-CN- application inhibited photolysis of CO-stimulated chemoreceptor discharge, revealing photometrically cytochrome a(592) as central in oxygen sensing. Control spectra in tissue from sympathetic and nodose ganglia did not show any cytochrome a(592) contribution. According to these results, cytochrome a(592) is assumed as a unique component of CB CCO, revealing in contrast to other cytochromes an apparent low PO2 and high CN- affinity, probably due to a shortcut of electron flow within CCO between Cu-A and cytochrome a3-Cu-B.