SUMMARYCarotid compression was performed in 32 patients with diagnosed cerebrovascular occlusive disease and in 33 elderly asymptomatic subjects. Recording of EEG, ECG, systolic blood pressure, and supraorbital plethysmography was performed simultaneously with the compression. In all the individuals included in this study, compression was considered effective as judged by a significant decrease in the amplitude of the plethysmogram on the side of the carotid being compressed. Eighteen percent of the asymptomatic subjects and 34% of the patients showed changes (the great majority unilaterally) in the EEG during carotid compression. The side of the EEG alteration showed a great tendency to coincide with the side of the carotid being compressed and not with the clinically more affected hemisphere. There was no correlation between EEG changes and changes in systolic blood pressure occurring during carotid compression. No complications were encountered. It is concluded that, when performed under careful observation of the EEG, the ECG, and continuous blood pressure recording, compression of the common carotid, applied at the base of the neck, is a safe test which gives an important indication of the functional condition of the large neck vessels and of the adequacy of some of the cerebrovascular autoregulatory mechanisms.