Type I cells of the carotid body play a major role in acid chemoreception. Extracellular acidosis causes membrane depolarisation, Ca²⁺ influx and neurosecretion in the type I cell. A previous study has shown that pH_i in the type I cell is very sensitive to changes of pH_o, and intracellular acidification is a key step in the signalling pathway for acid chemoreception. This thesis investigates the mechanism responsible for mediating acid influx during isocapnic extracellular acidosis. Type I cells were enzymically isolated from carotid bodies obtained from neonatal rats. pH_i was determined by microspectrofluorimetry, using pH-sensitive fluorescent dye carboxy-SNARF-1. My results show that there are two acid influx pathways. At resting pHi, Cl^--HCO₃^- exchange accounts for over 70% of acid influx in response to extracellular acidosis. The remaining 30% of acid influx is mediated by an unidentified mechanism, which does not require either Cl^- or HCO₃^-. I have also demonstrated that, the second pathway is an acid loading mechanism enhanced by a fall in pH_o, rather than an existing background acid loading unmasked by the inhibition of acid extruder. Although 200 µM DIDS inhibited Cl^-₀-free induced acid efflux mediated by reversed mode of Cl^--HCO₃^- exchange as well as the acid influx induced by alkali load, it had no effect on the acid influx in response to acid challenge. The difference in DIDS effect to block acid influx is probably due to difference in Cl^--HCO₃^- exchangers. I proposed that the Cl^--HCO₃^- exchange system in the type I cell comprises two distinct exchanger populations. One of them is DIDS sensitive and activated by high pH_i, while the other is DIDS insensitive and activated by low pH_o. The pH_i and pH_o sensitivity of both acid influx pathways have also been characterised. It is found that the unidentified HCO₃^--independent acid loading mechanism is activated by H₀⁺ while virtually pH_i-independent. In addition, the activity of Cl^--HCO₃^- exchange system is very sensitive to pH_i and pH_o, with pK_aⁱ and pK_a^o values close to resting pH_i and pH_o. Thus any shift of pH_i or pH_o from the normal resting range will produce significant changes in exchange activity, leading to changes in acid flux into the cell. In this way, the Cl^--HCO₃^- exchange system serves as a link for transducing acidic pH₀ into parallel acidification in pH_i in the type I cell.