Transition between phases in the respiratory cycle is an essential process of respiratory rhythm generation. Previous studies have shown that inspiratory termination is normally promoted by two pathways projecting onto the respiratory central rhythm generator, the pontine pneumotaxic centre and vagal afferents from pulmonary stretch receptors (reviews in Bianchi, Denavit-Saubié & Champagnat, 1995; von Euler, 1986). In vagotomized cats the spontaneously occurring inspiratory off-switch (IOS) can be delayed pharmacologically by dizocilpine, an antagonist of N-Methyl-d-aspartate (NMDA) receptors, resulting in an apneustic respiration (Foutz, Champagnat & Denavit-Saubié, 1988, 1989; Pierrefiche, Foutz, Champagnat & Denavit-Saubié, 1992). However, blockade of NMDA receptors does not suppress IOS induced by stimulation of peripheral sensory afferents (Foutz et al. 1989; Karius, Ling & Speck, 1991). The present study investigated whether the different sensitivity to NMDA receptor blockade of IOS of central or peripheral origin was due to different patterns of synaptic interactions on the target respiratory neurones of the ventral respiratory group (VRG). We recorded synaptic responses of respiratory neurones to stimulation of the superior laryngeal nerve (SLN) and vagus nerve, before and after induction of an apneustic respiratory pattern by dizocilpine. The effects of SLN stimulation on the respiratory rhythm have been extensively described (Berger, 1977; Iscoe, Feldman & Cohen, 1979; McCrimmon, Speck & Feldman, 1987), and it was shown in electrophysiological and anatomical studies that laryngeal afferents in cats terminate in the interstitial, medial, and ventrolateral subnuclei of the nucleus tractus solitarius (Kalia & Mesulam, 1980; Lucier, Egizii & Dostrovski, 1986; Bellingham & Lipski, 1992; Takagi, Umezaki & Shin, 1995). Stimulation of SLN afferents affects a wide population of inspiratory (I), post-inspiratory (PI) and expiratory neurones located in the VRG and its rostral-most part, the Bötzinger Complex (Czyzyk-Krzeska & Lawson, 1991; Jiang & Lipski, 1992), but the pathways through which these afferents innervate respiratory neurones remain to be determined. Some relay neurones of the SLN were found in the retrofacial nucleus, but these neurones do not seem to project to propriobulbar or bulbospinal respiratory neurones (Ezure, Oku & Tanaka, 1993). Within the respiratory neuronal network, PI neurones are of great interest because they can be activated by peripheral afferent stimulation (Remmers, Richter, Ballantyne, Bainton & Klein, 1986) and because their activity is profoundly depressed after pharmacological blockade of the spontaneous IOS mechanism (Pierrefiche et al. 1992; Haji, Pierrefiche, Takeda, Foutz, Champagnat & Denavit-Saubié, 1996b). Inspiratory (I) neurones are also important because they are the target of the IOS mechanism (Bianchi et al. 1995). Therefore we analysed the postsynaptic events occurring in I and PI neurones during the peripherally evoked IOS in eupneic and apneustic respiration induced by dizocilpine.