The action of four pharmacologically active polypeptides (vasopressin, oxytocin, bradykinin and neurokinin) on the pial arteries was studied in dogs anesthetized with pentobarbital sodium, and prepared for recording the arterial pressure and measurement of the diameter of the pial vessels. Twenty four experiments were carried out by using five groups of animals : one of these groups was used as control and the remaining four were used for studying each drug, administered intravenously. These experiments led to the following conclusions: 1) vasopressin does not determine significant modification on the diameter of the arteries of the pia mater, although has provoked irregular changes on the arterial pressure; 2) oxytocin determines a marked but evanescent pial vasoconstriction, without modifying the arterial pressure; 3) bradykinin increases the diameter of the pial arteries, this enlargement being related to the duration of its hypotensive effect; 4) neurokinin dilates the arteries of the pia mater, independently of some occasional change of the systemic blood pressure.