The petrosal ganglion (PG) neurones that innervate the carotid body, through the carotid sinus nerve, respond to exogenously applied ACh. About 60% of PG neurones in culture respond to ACh with dose-dependent inward currents, depolarization and action potentials, effects mimicked by nicotine and blocked by hexamethonium and mecamylamine. However, little is know about the identity of this nAChR. Immunohistochemical studies revealed that α7 and α4 nAChR subunits are present in the nerve endings and in the perikarya of cat PG neurones. To further characterize the receptors involved in PG neurones response to ACh, we studied the responses elicited in isolated petrosal ganglion neurones by selective agonists and antagonists of nAChRs. PG neurones were obtained from 10 adult cats anesthetized with pentobarbitone (40 mg kg^-1 i.p.). Animals were killed by an overdose of pentobarbitone. The cells were enzymatically dissociated, plated in Petri dishes and maintained at 38°C in water-saturated, 5% CO₂ in air atmosphere. The experimental protocol was approved by the Ethical Committees of both Faculties and met the guidelines of the Chilean National Fund for Scientific and Technological Development (FONDECYT). The currents evoked by ACh, cytisine (an alkaloid with micromolar affinity for α7 homomeric nAChRs) and its brome derivatives, 3-bromecytisine (3Br-cytisine; a selective agonist of α7 homomeric nAChRs), and 5-bromecytisine (5Br-cytisine; a selective agonist of α4β4 nAChRs), were measured using whole-cell voltage-clamp recordings at a holding potential of −60 mV. The agonists and antagonists, hexamethonium and α-bungarotoxin, were applied by superfusion under gravity from a pipette near the neurone surface. Pooled data of individual cells were fitted to a logistic curve [I = Inullmaxnull/1+ (EC₅₀/X)ⁿ], yielding correlation coefficients higher than 0.90 (p < 0.01) for all conditions studied. ACh evoked a fast inactivating inward current, whose amplitude showed a dose-dependent increase and was reversibly blocked by 1 μM hexamethonium. The ACh-sensitive neurones (10/16), also responded to cytisine, 3Br-cytisine and 5Br-cytisine with fast inactivating inward currents. The potency order of the agonists, according to their EC₅₀, was 3Br-cytisine >> ACh ≅ cytisine > 5Br-cytisine. Furthermore, 10 nM α-bungarotoxin blocked the responses induce by ACh in 5 out of 6 neurones, which electrical properties remained unchanged during the application of the antagonist. Our results suggest that most PG neurones respond to ACh by the activation of α7 nAChRs.