In 40 patients with unilateral occlusion of the internal carotid artery, using a transcranial Doppler device, blood flow velocity in the ipsilateral ophthalmic and middle cerebral artery was registered. During compression of the ipsilateral common carotid artery a decrease of ophthalmic artery flow velocity was noted in 39 patients (97.5%) and a decrease of middle cerebral artery flow velocity in 8 patients (20%). The average decrease of mean velocity in the middle cerebral artery was 4.5% of the initial value. During compression of the ipsilateral superficial temporal and facial arteries ophthalmic artery flow velocity decreased in 10 patients (25%) and no marked decrease of middle cerebral blood flow was noted. Conclusions. The external carotid artery in most of the patients with internal carotid occlusion is of no significance for cerebral blood supply, but it is the most important source of collateral blood supply to the eye. The maxillary artery, and not superficial temporal and facial as it seemed in periorbital Doppler examinations, is the branch of the greatest collateral value for the eye and brain.