The possible relationship between CO2 responsiveness and body mass in birds was explored using newly acquired ventilatory data from the barn swallow, Hirundo rustica, and the pigeon, Columbia livia, and that from the literature on four other species. Ventilatory responsiveness (%ΔV̇) of birds to 5% inspired CO2 is scaled to body mass to the 0.145 power (%ΔV̇∝ Mb0.145). A similar allometric relationship exists for data on 7 species of eutherian mammals taken from the literature (%ΔV̇∝ Mb0.130). The reduced responsiveness to CO2 in small birds and mammals may be related to an elevated hypoxic ventilatory sensitivity, as demonstrated in mammals (Boggs and Tenney, Respir. Physiol. 58: 245-251, 1984). These scaling relationships may reflect a mechanism for minimizing the inhibition of ventilation resulting from excessive loss of CO2 which thereby permits a higher hypoxic ventilatory response in small species. Other mechanisms, however, could include size related differences in mechanics or alveolar ventilation.