Summary: Activation of a cardiac chemoreceptor with serotonin elicits a reflex which includes changes in heart rate, contractile force, regional blood flow and hypertension. In six anaesthetised dogs we simultaneously recorded parasympathetic and sympathetic efferent traffic elicited during this cardiogenic reflex. The parasympathetic fibres were confirmed by reciprocal frequency changes with changes in blood pressure. The sympathetic fibre activity (anterior ansa subclavia) was attenuated or eliminated by ganglionic blockade or by clonidine. Whereas the phasic sympathetic multifibre discharge was only followed by a quiet period, the parasympathetic multifibre discharge was both preceded and followed by a quiet period. The sympathetic discharge preceded the parasympathetic discharge by 683 ± 170 ms. These autonomic efferent discharges were not elicited by administration of serotonin into the carotid artery, but were abolished by pretreatment with the serotonin antagonist, cyproheptadine. Cyproheptadine blockade could be overcome by increasing the serotonin concentration tenfold. This remarkable neural asynchrony has important implications concerning the electrical stability of the heart.