Mesenteric arteries and the hind-limb vasculature supplied by the sciatic artery were isolated from chickens 0–25 vity days after hatching and perfused with physiological saline. Mesenteric arteries gave only small responses to norepinephrine until the 2nd week after hatching when there was a striking increase in their reactivity. Hind-limb blood vessels were much more responsive to norepinephrine from the 1st day. The height of vasoconstrictor responses in hind-limbs showed no marked change with age but the duration of responses decreased as chickens grew older. The duration of responses to norepinephrine increased when perfusion temperature was lowered from 40 to 20 °C. Acetylcholine produced vasodilatation in mesenteric and hind-limb blood vessels. It is concluded that the receptor mechanisms for adrenergic and cholinergic control of hind-limb blood flow are functional at or before hatching. The increase in vascular reactivity to norepinephrine in mesenteric arteries coincides with reported increases in blood pressure and thermoregulatory ability of chickens and may contribute to these increases.