1. The activity of cardiac vagal efferent fibers was observed in fine strands obtained by splitting the cervical vagus of the dog anesthetized with morphine, chloralose and urethane. The responsiveness to sinus nerve stimulation was used as a criterion for identification of cardiac fibers.
2. The conduction rates of cardiac vagal fibers ranged from 4 to 12 m/sec, averaging about 8 m/sec.
3. Most cardiac vagal fibers were spontaneously active. Both spontaneous and reflexive discharges were inhibited during the inspiratory phase of spontaneous respiration.
4. The reflex discharge of a cardiac vagal fiber to sinus nerve stimulation was non-specifically inhibited when either the vagus, sciatic, brachial or saphenous nerve was simultaneously stimulated.