The steady-state discharge of fourteen arterial chemoreceptor preparations were recorded from the left cervical vagi of unidirectionally ventilated, pentobarbitone anaesthetized ducks. All were excited by both hypoxia and hypercapnia and these stimuli interacted multiplicatively, as they do in mammals. We located the receptive fields of three preparations by observing their responses to i.v. injections of 2,4-dinitrophenol before, during and after occlusion of various arteries. The responses of two preparations were consistent with their location in the ipsilateral carotid body, but the responses of one, containing two active fibres, suggested that its discharge originated in aortic bodies. The discharge of eleven preparations was not random, but came in short high frequency bursts. As stimulus intensity was increased by either hypoxia or hypercapnia the average number of impulses per burst decreased. We have shown that the arterial chemoreceptors of the duck are sensitive to both hypoxia and hypercapnia. Because the steady-state stimulus-response characteristics are essentially the same as those of mammals we suppose that both mammalian and avian chemoreceptors are excited by the same basic mechanism. We also show that ducks have active extra-carotid arterial chemoreceptors.