Catecholamine secretion from carotid body glomus cells is hypothesized to cause the hypoxia-induced increase in nerve activity. To test aspects of this hypothesis, tissue catecholamine and single-fiber nerve activity was measured from rat carotid bodies in vitro. Hypoxia (1-min duration, 0 Torr at nadir) caused a rapid increase in catecholamine release and nerve activity, consistent with the hypothesis, but repetitive hypoxias interspersed with short rest periods resulted in a much greater decline in catecholamine release than nerve activity. Furthermore, pretreatment with reserpine (24 h) nearly abolished catecholamine release, but nerve response was not different than untreated controls. These results suggest that catecholamine secretion is not causal to the increase in nerve activity of rat carotid body.