We compared regional blood flow in unanesthetized newborn lambs with that in adult sheep during acute, isocapnic hypoxic hypoxia [HH, 40-50% reduction of arterial O2 content (CaO2)]. The HH response in lambs and adults was qualitatively similar in heart, brain, and skeletal muscle, where flow increased; and in spleen, where it decreased. The response differed in skin and kidney, where flow decreased in lambs and was unchanged in adults, and in small intestine, where it was unchanged in lambs and increased in adults. Thus vasoconstriction during HH was less prominent in skin, kidney, and small intestine in adults. However, the trend toward lesser vasoconstriction in the adult cannot be attributed to a diminishing carotid chemoreflex and/or a more prominent vasodilatory lung inflation reflex because the same trend occurred during carbon monoxide hypoxia (COH). COH reduces CaO2 but stimulates neither the carotid chemoreflex nor, since hyperpnea is absent, the lung inflation reflex. Within each age group the responses to COH and HH were qualitatively the same. These data therefore provide no evidence for an active carotid chemoreflex in unanesthetized postnatal sheep. This is either because the peripheral circulatory effect of the chemoreflex is suppressed by the lung inflation reflex or, less likely, because the chemoreflex does not operate in the sheep at this level of HH.