The pattern of noradrenergic (NA) and acetylcholinesterase-positive (AChE) innervation in the internal carotid artery (ICA) and cerebral arterial tree was investigated in the duck. Cerebral perivascular NA and AChE nerves mainly enter the cranial cavity along the ICA. The NA nerves via the ICA originate exclusively in the axons within the sympathetic internal carotid nerve (SICN), while AChE nerves from the same vascular route have their major origin at the AChE nerve cells that are contained in the stem nerve bundle accompanying the SICN. In addition, a portion of AChE nerves in the internal carotid system (ICS) is derived from AChE nerve cells intrinsic to this cerebral arterial system. The major arteries of the duck ICS, as well as its ICA, were innervated by abundant NA and AChE nerves with approximately the same density. This finding is quite different from an unbalanced NA and AChE innervation in the corresponding arteries of the quail that is characterized by a markedly lesser density or lack of AChE nerves, suggesting that there is significant species difference in the cholinergic mechanisms for functioning of these arteries in the blood supply to the avian brain.