1. Inhibition and excitation of spontaneous phrenic nerve discharges in response to stimulation of the sinus, glossopharyngeal, aortic and superior laryngeal (SLN) nerves has been investigated in cats.2. The inhibition, in response to a single shock, had a latency of 5-10 msec and lasted for 20-40 msec; the response to SLN stimulation was the most prolonged.3. Excitation of phrenic motoneurones also occurred and was seen either before or after the end of the inhibition of the inspiratory burst and sometimes also during expiration.4. Intravenous strychnine blocked the inhibition.5. Intracellular recording from phrenic motoneurones showed that hyperpolarization was evoked by each nerve during the central phrenic depolarizing potential (CPDP) but only rarely in the interval between these potentials.6. When the CPDP was suppressed, hyperpolarization could sometimes be evoked.7. There were no changes in amplitude or time course of hyperpolarization during the passage of current through the cell membrane. No change in membrane conductance could be shown by passing current pulses.8. Raising the pressure in the carotid sinus also depressed phrenic activity and evoked a hyperpolarization whilst the CPDP was suppressed.9. Inspiratory interneurones in the brain stem were suppressed by nerve stimulation and by a rise in carotid sinus pressure.10. Expiratory interneurones in the brain stem were both excited and suppressed by electrical stimuli and unaffected by a change in carotid sinus pressure.