As part of our investigations on the chemosensory mechanisms in the rat carotid body, we are studying the physiology of the parenchymal glomus cells by the patch-clamp technique. Here we characterize a large-conductance chloride channel (approximately 296 pS) with random open and closed kinetics in inside-out patches of cultured glomus cells. The open-state probability (Po; mean = 0.61) was hardly affected by membrane potential (-50 to +50 mV) and cytoplasmic calcium (0-1 mM). Similarly, the channel did not appear to be regulated by cytoplasmic nucleotides (1 mM) or pH (6.5-8). Ion-substitution experiments yielded the following selectivity sequence: chloride greater than bicarbonate greater than sulfate greater than glutamate approximately sodium. Single-channel currents were reversibly reduced or blocked by anthracene-9-carboxylic acid (5-10 mM) but were unaffected by stilbene derivatives (0.5-1 mM), by furosemide (1 mM), and by 5-nitro-2-(3-phenyl-propylamino)benzoic acid (0.01 mM). Because these cultured glomus cells have been shown to express carbonic anhydrase, it is inferred that the chloride channels may play an important role in the physiology of glomus cells by aiding in the regulation of pHi and the resting potential via bicarbonate and chloride permeability.