The relationship between oxygen pressure (PO2) in the carotid body and carotid sinus nerve discharge was evaluated in the isolated perfused/superfused cat carotid body using the oxygen-dependent quenching of phosphorescence. Images of phosphorescence intensity arising from Pd-coproporphyrin within the microcirculation of the carotid body provided measurements of intravascular PO2. These measurements were substantiated by determining phosphorescence life-time. The carotid body was perfused in the isolated state via the common carotid artery with N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid-buffered Tyrode solution, pH 7.4, at a constant pressure of 80 mmHg. Superfusion was maintained with similar media equilibrated with 100% argon. PO2 in the exchange vessels was markedly less than that in the perfusate entering the carotid artery, 23 +/- 3 and 45 +/- 3 Torr for normoxic (111 +/- 15 Torr) and hyperoxic (345 +/- 72 Torr) perfusates, respectively. Chemosensory discharge rose slowly in response to a brief interruption of perfusate flow as PO2 steadily declined from either of these capillary PO2 values to approximately 10 Torr. Between approximately 10 and 3 Torr, chemosensory discharge increased strikingly, concomitant with an enhanced rate of oxygen disappearance, from -36 +/- 4 to -69 +/- 13 (92% change) and -28 +/- 3 to -48 +/- 3 (71% change) Torr/s for normoxic and hyperoxic perfusates, respectively. As PO2 fell below approximately 3 Torr, oxygen disappearance slowed and neural activity decayed. Thus the relationships between microvascular PO2 and chemosensory discharge and between oxygen disappearance and neural discharge suggest that oxygen metabolism in the carotid body determines the expression of oxygen chemoreception.