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The carotid body (CB) located at the carotid artery bifurcation informs the central nervous system about arterial p02 changes. The CB response to hypoxic stimuli includes augmented transmitter release and increased generation of action potentials in the carotid sinus nerve (CSN). Until now the molecular and cellular mechanisms of the oxygen sensing signal cascade are not clear. Cellular oxygen sensing processes are believed to be initiated by specialised cytochromes (Bunn and Poyton, 1996; Lopez-Barneo et al. 2001). In recent studies (Lahiri and Acker, 1999; Lahiri et. al 1999) simultaneous recordings of rat CB light absorption spectra and chemoreceptor discharge were used to characterize heme proteins as primary oxygen sensor. Redox spectra were deconvoluted by means of redox differential spectra of isolated non mitochondrial cytochrome b558 and mitochondrial cytochromes c550, b563 and the cytochrome c oxydase (CCO) peaking at 603 nm. Mathematically deconvolution fitted very closely to experimental spectra in the wavelength range between 520 and 570 nm confirming the contribution of cytochromes b558, c550 and b563 in the carotid body tissue. However, the wavelength range between 570 and 620 nm normally dominated by cytochrome aa3 of the CCO (absorbance peak at 603 nm) was fitted only partly indicating an additional component.