1. Respiratory responses to altered P(a, O2) at constant P(a, CO2) and altered P(a, CO2) at constant P(a, O2) were measured in fourteen unanaesthetized ducks 5-9 weeks old.2. All the ducks responded to the inhalation of a few breaths of 100% O(2) with a fall in V(E) of between 11 and 17% of control, and to the reduction in P(a, O2) from control levels (93-101 mm Hg) to 38-47 mm Hg with an average increase in V(E) of 190% of control which was potentiated by raising the level of CO(2).3. Ventilation approximately doubled when P(a, CO2) was raised to 55 mm Hg, the greatest sensitivity to changes in P(a, CO2) being over the range 40-45 mm Hg.4. Carotid body denervation was carried out in nine ducks and the respiratory responses were modified. The fall in V(E) with inhalation of 100% O(2) was abolished in all ducks and the rise in V(E) with hypoxia was abolished in six. The increase in V(E) as P(a, CO2) was altered in steps was unaffected but the rate at which V(E) increased in response to 4% CO(2) was markedly reduced.5. It is concluded that the chemoreceptor regulation of respiration in ducks is similar to that observed in non-diving animals.