1. We examined the upper airway (UA) motor response to upper airway negative pressure (UANP) in the rat. We hypothesized that this response is mediated by superior laryngeal nerve (SLN) afferents and is not confined to airway dilator muscles but also involves an increase in motor drive to tongue retractor and pharyngeal constrictor muscles, reflecting a role for these muscles in stabilizing the UA. 2. Experiments were performed in 49 chloralose-anaesthetized, tracheostomized rats. Subatmospheric pressure in the range 0 to -30 cmH(2)O was applied to the isolated UA. Motor activity was recorded in separate experiments from the main trunk of the hypoglossal nerve (XII, n = 8), the pharyngeal branch of the glossopharyngeal nerve (n = 8), the medial and lateral branches of the XII (n = 8) and the pharyngeal branch of the vagus (n = 8). Afferent activity was recorded from the whole SLN in six experiments. 3. All UA motor outflows exhibited phasic inspiratory activity and this was significantly (P < 0.05) increased by UANP. Tonic end-expiratory activity increased significantly in response to pressures more negative than -20 cmH(2)O. Bilateral section of the SLN also increased (P < 0.05) motor activity and abolished the responses to UANP. Electrical stimulation of the SLN inhibited inspiratory XII activity. SLN afferents were tonically active and inhibited by UANP. 4. We conclude that UANP causes a reflex increase in motor drive to pharyngeal dilator, tongue retractor and pharyngeal constrictor muscles via afferent fibres in the SLN. Tonic activity in SLN afferent fibres at zero transmural pressure exerts a marked inhibitory effect on UA motor outflow.