ABSTRAm. We characterized changes in the vagal canine heart, the vagal chronotropic response increases in magchronotropic response during 3-min trains of vagal stimu-nitude as vagal stimulation frequency is increased. However, lation at 3, 5, and 8 Hz in anesthetized, chemically sym-while the vagal chronotropic response increases, the fade ratio (a pathectomized (6-hydroxydopamine) newborn canines measure of the attenuation of the vagal chronotropic response (