Rheoencephalographic investigations (REG) was carried out in patients with intracranial aneurysms and in healthy subjects. The purpose of the study was to obtain approximate data on the efficiency of collateral circulation of the brain before ligation of the internal carotid artery. For this purpose REG was associated with common artery compression test. It was found that this combined examination gives good prediction of the efficiency of collateral circulation after ligation of one carotid artery. This method is more sensitive than the Matas test combined with EEG used previously.