1. The carotid sinus nerve was electrically stimulated in dogs anaesthetized with chloralose. Stimuli (1 ms, 1-10 V, less than or equal to 1 Hz) evoked responses in single cardiac efferent fibres dissected from the cervical part of the vagus nerve. The mean latencies of these responses varied, from fibre to fibre, between 30 and 120 ms. 2. Stimuli given during the expiratory phase of the respiratory cycle evoked vagal responses with a shorter latency than similar stimuli given only during the inspiratory phase of the respiratory cycle. 3. Following the vagal response to carotid sinus nerve stimulation a period of inhibition of vagal activity, lasting 100-150 ms, occurred. Refractoriness of the responding vagal motoneurone following an action potential could not account for this post-excitatory depression. 4. The inhibitory effects of electrical stimulation of the carotid sinus nerve were further studied by applying pairs of similar electrical stimuli to the carotid sinus nerve. The second stimulus of a pair had to be given 80-100 ms after the first to evoke a second response. 5. Trains of electrical stimuli (30-100 Hz) were also studied. At low frequencies the inhibitory effect of successive stimuli on vagal responses became less marked, but at higher frequencies only the first and last stimulus of the train reliably evoked responses. For trains of stimuli at both low and high frequencies, the last stimulus of the train evoked a vagal response, which was succeeded by a period of inhibition of vagal firing: this inhibition was then followed by further excitation before vagal discharge returned to resting levels.