The effects of ionophoretic application of acetylcholine (ACh) and cholinergic antagonists to respiratory neurones of the nucleus ambiguus-retroambigualis complex were studied in cats. 2. Acetylcholine was ineffective when applied to inspiratory neurones (27/31) but inhibited the spontaneous activity of a significant population of expiratory neurones (21/39). 3. The expiratory neurones inhibited by ACh included neurones with axons projecting to the vagus or spinal cord, as well as neurones not antidromically activated from either of these sites. 4. The cholinergic inhibition of expiratory neurones was blocked by atropine (6/9) but dihydro-beta-erythroidine was ineffective. In seven expiratory neurones, ionophoresis of atropine alone evoked firing during the normal period of silence of the neurone (inspiration). 5. It is concluded that a significant population of expiratory neurones receive an active inhibition during inspiration and this involves a cholinergic mechanism. The implications of these observations for the synaptology of central respiratory control and modifications of the vagal outflow to the heart are discussed.
