In anaesthetized, paralysed and artificially ventilated dogs, activities were recorded from the phrenic nerve and from respiratory units within the nucleus tractus solitarius (NTS) and the nucleus ambiguus (NA). Prolonged stimulation of peripheral chemoreceptors by almitrine (50 micrograms X kg-1 i.v.) induced an increase in the firing rate of the R alpha and R beta inspiratory neurones of the NTS. Some expiratory neurones were found in the NTS and their firing rate was also enhanced by almitrine. In the NA, the respiratory neurones have active phase-firing patterns that can be quite different: the late peak inspiratory or expiratory neurones have linearly augmenting discharges with a steeper cessation. In contrast the early burst inspiratory or expiratory units start to fire with the onset of their active phase, rapidly achieve peak firing frequency and then the firing frequency declines through the remainder of the active phase. Almitrine had been found to activate these neurones particularly the expiratory neurones of the N.A.