The roles played by GABA_A and glycine receptors in inspiratory-expiratory motor co-ordination and in tonic inhibitory regulation of expiratory motor activity were studied using brainstem-spinal cord (-rib) preparations from neonatal rats. Inspiratory activity was recorded from the C4 ventral root. Expiratory activity in internal intercostal muscle, internal oblique muscle or T13 ventral root was evoked by a decrease in perfusate pH from 7.4 to 7.1 (i.e. from normal to low pH conditions) and was limited to the first part of the expiratory phase. Under low pH conditions, bath application of 10 µM bicuculline, a GABA_A receptor antagonist, caused the inspiratory burst to overlap the expiratory burst in 2/7 preparations. Overlapping of the expiratory burst with the inspiratory burst was observed in 7/7 preparations made under 10 µM bicuculline. Furthermore, such preparations exhibited expiratory bursts under bicuculline-containing normal pH conditions. Local application of 10 µM bicuculline to the brainstem under normal pH conditions evoked expiratory bursts, some of which overlapped the inspiratory bursts. Picrotoxin, another antagonist of the GABA_A receptor, had similar effects. Under normal pH conditions, application of strychnine (0.2- 2.0 µM; a glycine receptor antagonist) to the brainstem did not evoke expiratory bursts. On subsequent application of strychnine-containing low pH solution, expiratory bursts were evoked and some (0.5 µM) or all (2.0 µM) of these overlapped the inspiratory burst. Simultaneous application of picrotoxin and strychnine to the brainstem evoked expiratory bursts that overlapped the inspiratory bursts and a subsequent decrease in perfusate pH to 7.1 increased the frequency of the respiratory rhythm. It was a characteristic finding that the duration of the expiratory burst exceeded that of the inspiratory burst under control low pH conditions. This remained true during concurrent blockade of GABA_A and glycine receptors. The results suggest that in the in vitro preparation from neonatal rats: (1) GABA_A and glycine receptors within the brainstem play important roles in the co-ordination between inspiratory and expiratory motor activity, (2) tonic inhibition via GABA_A receptors, but not glycine receptors, plays a role in the regulation of expiratory motor activity and (3) inspiratory and expiratory burst termination is independent of both GABA_A and glycine receptors.