The authors studied the action of the l-hexyl-3,7-dimethylxanthine on the blood pressure and on the caliber of the pial vessels of the dog. In 15 dogs anesthetized by sodium penthobarbital (30 mg/kg) the arterial blood pressure was recorded with a mercury manometer. The pial vessels were observed through a window made in the temporal bone with the aid of binocular stereoscopic microscope. For photography one of the oculars of the microscope was replaced by an appropriate camera. The procedure was that proposed by Forbes4 modified by Melaragno5. Groups of 4 animals received respectively 15, 30 and 60 mg/kg of the drug by the oral route and were observed until 240 minutes. The other three animals were used as control and observed by the same interval of time, receiving only the anesthetic. The oral administration of the l-hexyl-3,7-dimethylxanthine induced marked relaxation of the pial vessels without concomitant fall of the arterial blood pressure, in spite of the doses of 15 mg/kg.