It has been suggested that avian intrapulmonary CO2-sensitive receptors (IPC) may be capable of monitoring the rate and extent of CO2 washout from the lung during spontaneous breathing. The purpose of this study was to analyse IPC discharge activity (using computerised bin-averaging and counting techniques) in spontaneously breathing domestic fowl when VT was elevated from resting levels. This was accomplished by administration of almitrine (2 mg X kg-1 i.v.), a respiratory stimulant drug that has been shown to have a specific long-lasting stimulatory action on carotid body chemoreceptors. Unanaesthetized decerebrate chickens were tracheotomized and single unit activity was recorded from 14 IPC. When VT progressively increased following administration of almitrine (with little or no change in TI or TE), IPC activity increased in a linear relationship with the increased VT. IPC activity in expiration was also increased, and the delay period before the onset of IPC discharge in inspiration was shortened. It is concluded that IPC discharge is increased when VT is elevated in the spontaneously breathing chicken and hence the IPC are capable of monitoring the extent of each ventilatory effort. It is well known that IPC have strong inhibitory effects on ventilatory motor output and conceivably they could originate reflexogenic information to the respiratory centres in response to intrapulmonary PCO2 changes. The latter could arise from changes in CO2 delivery by the mixed venous blood or from changes in the extent of CO2 washout with each breath.