This study was designed to investigate the effects of halothane on the electrical activity of the respiratory muscles in rats. Indeed, halothane is known to reduce end-expiratory lung volume, both in man and in animal; this may be due to altered activity in the respiratory muscles. The electrical activity of the diaphragm, parasternal, intercostal and abdominal muscles were recorded using intramuscular electrodes in ten rats weighing between 400 and 450 g each. The rats were prepared under light halothane anaesthesia (tracheostomy, laparotomy, electrode positioning, plaster of Paris cast to impede leg movements). They were placed prone in a 20 1 plexiglass chamber, and allowed to awake. Thereafter halothane was vaporized in this chamber at a known concentration, until the animals no longer reacted to tail pinching. The measurements were carried out during the awake state, and under 2 vol % halothane. Muscle tone was assessed by the thickness of baseline inspiratory muscle activity at the end of expiration (maximally amplified raw electromyographic signals). The measurement of phasic electrical activity was carried out using peak inspiratory integrated electromyographic signals. Under halothane, phasic activity of the diaphragm was slightly reduced (p less than 0.05), whereas tonic activity remained unchanged. Parasternal intercostal muscle activity, both tonic and phasic, was also decreased during halothane anaesthesia (p less than 0.001). No phasic activity occurred in the abdominal muscles, but muscle tone was reduced during halothane administration (p less than 0.01). In rats, the decrease in intercostal muscle tone under halothane anaesthesia could play a major part in the fall in functional residual capacity.(ABSTRACT TRUNCATED AT 250 WORDS)