Single-unit extracellular potentials from CO2-sensitive intrapulmonary chemoreceptors (IPC) were recorded with tungsten microelectrodes from the nodose ganglia of unidirectionally, artificially ventilated White Leghorn cockerels. The responses of IPC to step and sinusoidal forcing functions of fractional concentration of carbon dioxide (FCo2) indicate that 1) about two-thirds of IPC are bidirectionally rate sensitive, 2) delay to forcing varies from essentially zero to several times larger than the time of nitrogen gas passage through the lung and, on the average, is significantly shorter to step decrease than step increase in fco2, and 3) IPC with irregular discharge intervals are more rate sensitive and respond to higher frequencies than IPC with regular interspike intervals. The physical and chemical properties of the "receptor-lung" complex are probably important in determining the dynamic responses of IPC by partitioning of CO2 from other gases within the lumen of the parabronchus. IPC are located along the length of the bronchial gas exchange areas of the avian lung and transmit information to the CNS concerning the mean level, rate, and direction of change in the partial pressure of carbon dioxide (PCo2).