Carotid body (CB) responses to hypoxia are low at birth and increase over time to mature responses. Using an in vitro rat CB-carotid sinus nerve (CSN) preparation, Kholwadwala and Donnelly (1992) demonstrated that the CSN activity in response to hypoxia increased from low levels to a robust adult response after two weeks. This time course of maturation was paralleled by an increase in TASK channel sensitivity to hypoxia in the O2-sensing Type I cells (Kim et al, 2011). The mechanism(s) which underpins this increase in O2-sensitivity has not been addressed. However, a body of evidence suggest mitochondria in Type I cells are critical for O2-sensing by the CB. Here the hypothesis that changes in mitochondrial organization and rate of oxidative phosphorylation may account for the development of O2-sensitivity in CBs was examined. Type I cells were isolated from juvenile (4-6 days) and mature (14-16 days) rat carotid bodies, surgically removed during isofluorane (4%) anaesthesia. Cells were stained with Mitotracker and Celltracker, imaged using a DeltaVision system (Applied Precision) and analyzed for volume using the program Imaris XT (Bitplane). Total mitochondrial volume in juvenile Type I cells was 100.3 ± 13.6 µm3 (S.E.M, n = 23) and this was significantly smaller in mature cells, 22.6 ± 3.4 µm3 (S.E.M, n = 17, P<0.00003, unpaired Students t-test). This decrease in mitochondrial volume was not observed in non-oxygen sensing cells (superior cervical ganglion). Rate of oxidative phosphorylation was assessed by measuring the conversion of C12-resazurin to fluorescent resarufin using an Olympus FV1000MPE 2-photon confocal microscope. Resarufin was excited at 870nm and emission recorded at 587 nm. No significant differences were observed in the rate of production of resarufin between juvenile (n = 142) and mature (n =57) Type I cells. Resarufin production was abolished in the presence of cyanide (2mM). These data demonstrate significant reductions in the mitochondrial content of Type I cells during development. This decrease in number does not cause a reduction in the rate of production of resarufin indicating that the smaller number of mitochondria may have increased their rate of oxidative phosphorylation. It has been observed that the Km for oxygen in mitochondia is dependent on energy state (Petersen et al, 1974; Krab et al, 2011). Thus by increasing the rate of oxidative phosphorylation it is possible that mitochondria in mature Type I cells become more susceptible to inhibition by hypoxia than their juvenile counterparts. Consequently, the development of O2-sensitivity in rat carotid body Type I cells may be attributed to mitochondrial plasticity during the first two weeks of life.