We examined the reflex change in refractory period (RP) of the left ventricle produced by stimulation of the central end of the caudovagal nerve (CVS). RP was recorded in the region of distribution of the left anterior descending coronary artery. During the determination of RP, hearts were paced at cycle lengths between 244 and 315 msec. Initially, the tonic effects of removing vagal and sympathetic influences were determined. Neither right nor left vagotomy nor cardiac sympathectomy (symx) affected RP; however, bilateral vagal block decreased RP by 3 msec, whereas bilateral symx increased RP by 7 msec. Atropine increased RP by 2 msec, probably via inhibition of muscarinic receptors in ganglia. After determination of each of the tonic influences, CVS was performed with pulses of 35-50 Hz, 0.6-1.0 msec in duration, and 1.6-4.0 V. CVS increased RP by 8 msec. Right vagal cold block abolished the response, but left vagal block had no effect. Left symx had no effect on the response, but right symx limited it to 6 msec. Bilateral symx was similar to right symx alone. Subsequent atropine administration blocked 4 msec of the remaining response. Atropine given before symx limited the increase in RP due to CVS to 4 msec. After subsequent right symx, the increase in RP was only 1 msec. Left symx after atropine decreased the response only 1 msec. Total symx after atropine abolished the response. The degree of initial sympathetic activity had no effect on the RP response. We conclude that: (1) the afferent limb of the reflex lies in the right vagus; (2) the dominant efferent reflex path lies in the vagi; (3) the sympathetics have a significant role only when the vagi are blocked; (4) when active, the efferent sympathetic paths course mostly through the right side; (5) the vagi do not require existing sympathetic activity to exert an effect.Circ Res 46: 378-386, 1980