Previous studies have demonstrated that unmyelinated fibers in the cat carotid sinus nerve (CSN) are capable of inhibiting chemoreceptor activity from the carotid body. While the mechanism(s) underlying this phenomenon are unknown, it has been postulated that carotid body catecholamines may be involved in mediating this inhibitory effect. In this study, the in vitro cat carotid body-CSN preparation was used to investigate the effects of CSN stimulation on the release of [3H]catecholamines from the carotid body. The results showed that labeled catecholamines were released from this organ only when C-fibers were recruited by nerve stimulation.