We studied discharge frequencies of 12 intrapulmonary chemoreceptors in the paleopulmonic lung of an emu (Dromiceius novaechollandiae) during unidirectional, artificial ventilation when step changes and static CO2 concentrations were given. Discharge frequency in afferent neurons from the receptors increased as intrapulmonary CO2 decreased. The response of the receptors to various static intrapulmonary CO2 concentrations was similar to that previously demonstrated for the duck and chicken. The median sensitivity of the emu receptors at one-half maximal discharge was 4.6 imp-(sec-0.01 F1CO2)-1. Discharge frequencies altered phasically in nine of 12 receptors when step changes in CO2 of 3.3% at 1.6 Hz were given in the unidirectional gas stream while four of 12 modulated their discharge when the CO2 changes were as rapid as 3.2 Hz. Hence, some receptors can respond to rapid fluctuations in CO2 in their microencironment. We conclude that intrapulmonary chemoreceptors in the paleopulmonic lung of the emu exhibit similar characteristics to those in birds that possess varying amounts of neopulmonic parabronchi, such as the duck and chicken.