An ultrasonic Doppler method was used to monitor flow velocities in basal cerebral arteries in 21 patients with spontaneous subarachnoid hemorrhage (SAH). The time course of vasospasm as evaluated by this technique was similar to that reported in angiographic studies. In 82% of the patients an increase in intracranial velocities to 120 cm/s or more was found during the second or third week after hemorrhage. (Normal value 62 cm/s). Arterial narrowing giving rise to velocities above 200 cm/s was classified as severe spasm. This occurred in 42% of the cases, and a significant decrease in flow velocity in the extracranial carotid artery was found in this group.