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Carotid body (CB) type I cells sense the blood Po 2 and generate a nervous signal for stimulating ventilation and circulation when blood oxygen levels decline. Three oxygen-sensing enzyme complexes may be used for this purpose: 1 ) mitochondrial electron transport chain metabolism, 2 ) heme oxygenase 2 (HO-2)-generating CO, and/or 3 ) an NAD(P)H oxidase (NOX). We hypothesize that intracellular redox changes are the link between the sensor and nervous signals. To test this hypothesis type I cell autofluorescence of flavoproteins (Fp) and NAD(P)H within the mouse CB ex vivo was recorded as Fp/(Fp+NAD(P)H) redox ratio. CB type I cell redox ratio transiently declined with the onset of hypoxia. Upon reoxygenation, CB type I cells showed a significantly increased redox ratio. As a control organ, the non-oxygen-sensing sympathetic superior cervical ganglion (SCG) showed a continuously reduced redox ratio upon hypoxia. CN - , diphenyleneiodonium, or reactive oxygen species influenced chemoreceptor discharge (CND) with subsequent loss of O 2 sensitivity and inhibited hypoxic Fp reduction only in the CB but not in SCG Fp, indicating a specific role of Fp in the oxygen-sensing process. Hypoxia-induced changes in CB type I cell redox ratio affected peptidyl prolyl isomerase Pin1, which is believed to colocalize with the NADPH oxidase subunit p47 phox in the cell membrane to trigger the opening of potassium channels. We postulate that hypoxia-induced changes in the Fp-mediated redox ratio of the CB regulate the Pin1/p47 phox tandem to alter type I cell potassium channels and therewith CND.