We have studied the intracranial cerebral circulation in 6 patients with bilateral ischaemic lesions of the internal carotid artery in the extracranial segment (2 significant bilateral stenosis cases; 1 case with bilateral thrombosis and 3 cases of unilateral thrombosis and significant contralateral stenosis). All the patients were males, their age being between the 5th and 8th decade. In a single case the neurological examination showed secondary left hemiplegia and recent right paresis of remittent type, while the other 5 patients had only transitory ischaemic attacks with hemiparesis or transitory aphasia. The lesions were revealed by means of duplex system echotomography (Aloka-Hellige Model SSD 630) and spectral analysis of Doppler signal (Vasoscan-Sonicaid) and they were confirmed later by bilateral carotid arteriography in all patients. The intracranial circulation was also followed up by non-invasive methods, making use of spectral-analysis of the Doppler signal with pulsed wave on TC-2 64-B apparatus. The cases studied by us, which present pathogenic situations more rarely encountered, have shown that none of them observed a 'mathematical model' of compensation of blood flow (BF). More exactly, 2 patients with the same type of lesions and topography did not have a unique model of compensation of BF. It seems that both the possibilities of individual self-regulation of cerebral BF and the extracerebral factors, especially those belonging to cardiac activity, are decisive in the compensatory activity of cerebral circulation, while the modalities in which this is accomplished depend chiefly upon the functional condition of the collateral arteries as a whole.