In 13 hypertensive patients taking the beta-receptor blocker atenolol, EEG and behaviour effects were studied. Using a single-blind sequential trial design, a 5-d placebo period was followed by a 6-d atenolol treatment period (100 mg/d). During either treatment period, two examinations yielded data derived from neurophysiological, test-psychological, and cardiovascular measurements. The statistically significant changes due to atenolol therapy indicated an improvement of vigilance, a smooth anxiolytic and tranquilizing effect, and at least no negative influences on discriminatory reaction time and concentration ability. The primary therapeutic effects were significant falls in systolic and diastolic blood pressure as well as in pulse rates. The study is still in progress and will be completed after 20 patients are reached. The data will then be reanalyzed in order to get further insight into CNS effects of a beta-blocking therapy concerning favorable vs. poor drug responders.