Hindbrain mechanisms generating the respiratory rhythm in chicks were analysed. In vivo, ventilation and intercostal muscle activity were recorded in chicks (1 and 2.5 weeks-old), vagotomized and treated with the NMDA receptor blocker MK-801 (dizocilpine). In vitro, synaptic transmission from vagal to second-order sensory neurones was studied in the nucleus of the solitary tract, using whole-cell recordings in slices. Vagal afferents were found to act through GABAergic synapses and control two hindbrain systems: a dizocilpine-sensitive control system and a rhythm generator. Although this organization is the same as in mammals, after vagotomy entirely different respiratory patterns emerge: (i) expiratory-inspiratory efforts triggered by the rhythm generator and (ii) periods of apnoea produced by the dizocilpine-sensitive system.