Intracellular recordings were made from caudal medullary expiratory neurones in pentobarbitone-anaesthetized, vagotomized and artificially ventilated cats. The sample consisted of thirty-three bulbospinal neurones and seven neurones which were not antidromically excited from either the spinal cord (C2-C3) or vagus nerve. Their rhythmic activity consisted of an alternating inspiratory hyperpolarization due to Cl(-)-dependent inhibitory post-synaptic potentials (i.p.s.p.s) (Mitchell & Herbert, 1974) and an expiratory depolarization. The precise shape of the expiratory depolarizing wave varied within a given neurone depending on the over-all pattern of respiration. This variation extended from a smoothly developing depolarization, continuous throughout its course, through an intermediate state in which depolarization proceeded in two stages with a definite transition between them, to a final state in which the early part of expiration was occupied by a distinct hyperpolarizing component to the membrane potential trajectory. Under conditions of a brisk phrenic nerve discharge, these variations in the shape of the membrane potential profile were related to the time course and intensity of post-inspiratory discharge in the nerve. However, other factors (depth of anaesthesia and stimulation of laryngeal receptors) could influence the time course of the membrane potential profile of expiratory neurones independently of post-inspiratory phrenic discharge. In five of fifteen neurones which were tested, early expiration was occupied by a rapidly developing, decrementing wave of Cl(-)-dependent i.p.s.p.s (post-inspiratory i.p.s.p.s). These i.p.s.p.s were present only under conditions of a strong phrenic rhythm (large amplitude, fairly rapid phrenic discharge). They became weaker and ultimately disappeared when the level of anaesthesia was deepened and the phrenic rhythm became slower. Under these conditions, the post-inspiratory wave of i.p.s.p.s could be restored by stimulation of the superior laryngeal nerve. Adequate stimulation of presumed 'irritant' laryngeal receptors elicited post-inspiratory i.p.s.p.s in seven of ten neurones tested which initially showed either no post-inspiratory i.p.s.p.s or possibly just a weak pattern. In ten of fifteen neurones tested, the responses to current injection revealed clear differences in membrane potential behaviour in early and late expiration, which became intensified following stimulation of the superior laryngeal nerve.(ABSTRACT TRUNCATED AT 400 WORDS)