An ocular pneumoplethysmograph was devised to measure the systolic pressure in the ophthalmic artery and to obtain calibrated pulse volume changes from the ocular globe, with simultaneous recording from both eyes. The instrument is selfcontained and portable. It can be applied by paramedical personnel, and the graphic recording is readily interpreted by the physician. It has proved to be 100% accurate in noninvasively detecting unilateral carotid occlusion or preocclusive stenosis. This technique, when combined with carotid compression, has been shown to be most valuable in assessing the tolerance of a cerebral hemisphere to proximal carotid occlusion. It was used to evaluate a wide variety of carotid arterial lesions. Elective carotid resection without graft replacement and urgent and elective carotid ligations were performed, based on the preoperative prediction that such procedures would be tolerated without neurological injury. The instrument and the technique of its application are simple, safe, and accurate.