• Nitroglycerin is widely used as a safe and effective dilator of the large coronary arteries in patients with ischemic heart disease. Little is known, however, about its effects on the intracranial arteries. We compared, in the monkey, the effects of intravenous (IV) nitroglycerin, alone and in combination with phenylephrine hydrochloride, on the diameter of the intracranial portion of the internal carotid artery, the middle cerebral artery, and the lingual branch of the external carotid artery; IV nitroglycerin increased the diameter of all three. These arteries remained dilated even when phenylephrine was added to the infusion to raise the mean arterial pressure by up to 29% above control values.