1. The effects of severe arterial hypoxia on the blood flow in the portal vein, and in kidney, muscle and skin beds have been determined in normal unanaesthetized rabbits, in animals without functioning autonomic effectors, and in rabbits with section of the carotid sinus and aortic nerves.2. The resting blood flows in the above regions were not significantly different in the three groups.3. The susceptibilities of the various beds to the local dilator effects of arterial hypoxia (assessed from the responses of animals without functioning autonomic effectors) were markedly different; vasodilatation was by far the greatest in the portal bed, followed in order by the renal, skin and muscle beds.4. Section of the carotid sinus and aortic nerves completely abolished reflex activity, and the pattern of peripheral blood flow changes was similar to that of animals without functioning autonomic effectors. The findings suggest that the arterial chemoreceptors are the primary afferent source of reflex control of the peripheral circulation in arterial hypoxia.5. In normal animals with intact reflexes there was sustained vasoconstriction throughout the treatment period in the portal and renal bed. The net vasomotor effects in muscle and limb skin were small owing to the operation of a number of factors, which opposed the effects of reflexly increased sympathetic nerve activity.