1. The effects of primary tissue hypoxia induced by the inhalation of small concentrations of carbon monoxide in air on the distribution of blood flow in the portal, renal, muscle and skin beds have been studied in normal unanaesthetized rabbits, in animals without functioning autonomic effectors (‘de‐efferented’ rabbits) and in animals with section of the carotid sinus and aortic nerves (‘de‐afferented’ rabbits). 2. The pattern of blood flow distribution during CO hypoxia was similar in ‘de‐efferented’ and ‘de‐afferented’ animals, suggesting that the effects in the latter were determined by local mechanisms. The susceptibility of the various beds to the local dilator effects of CO hypoxia was markedly different, the greatest dilator effects being observed in the portal bed, followed by skin, kidney, and muscle. The pattern is somewhat different from that observed in arterial hypoxia. 3. In this type of hypoxia the arterial baroreceptors are probably the main afferent source of reflex activity. In normal animals reflex constrictor effects affect the portal and renal beds most, ‘moderating’ the local dilator effects of hypoxia in these beds. In muscle there is vasodilatation, probably the result of adrenaline secretion, but the response in skin is largely determined by the local effects of hypoxia. The total orthosympathetic activity evoked in this type of hypoxia appears to be less than in severe arterial hypoxia.