Since nucleotides have been postulated as transmitters between glomus cells and chemosensory nerve endings in the carotid body, we studied the effects of their application to the petrosal ganglion, where the perikarya of carotid (sinus) nerve are located. Cat petrosal ganglia were superfused in vitro, while electrical activities of their peripheral processes (carotid nerve and glossopharyngeal branch) were recorded simultaneously. Adenosine triphosphate (ATP) evoked dose-dependent bursts of impulses in carotid nerve, while those in glossopharyngeal branch were less intense and consistent. Adenosine monophosphate was less effective than ATP. ATP-induced carotid nerve responses presented no temporal desensitization and persisted after applying P(2Y) receptor blocker Reactive Blue 2 to the ganglion. The results indicate that ATP has an excitatory effect on the perikarya of the population of petrosal ganglion neurons projecting peripherally through the carotid nerve.