Excitation of a cardiac chemoreceptor with 5-hydroxytryptamine (serotonin) produces a complex autonomic reflex which includes hypertension, changes in heart rate and contractile force, and disturbances of AV conduction. This study examines the afferent and efferent neural pathways of this autonomic reflex in 60 anesthetized dogs. We used cooling and sectioning techniques in 40 of these, and in 20 others recorded afferent neurograms. The most common afferent pathways for the reflex were found in the left and right recurrent cardiac nerves. No preferential efferent routes to the heart were found, although the nature of the reflex cardiac response could be altered by specific nerve interruption. Cyproheptadine (1 mg/kg iv) regularly abolished both the reflex and the chemoreceptor afferent neural traffic, but injection of a 10 times higher concentration of serotonin (1 mg/ml) readily surmounted the blockade and restored the chemoreceptor neural traffic. Thus cyproheptadine interdicts the reflex at the site of its initiation.