Subtle changes of adaptive behaviour under lithium treatment may possess interest not only as side-effects, but also as a possible explanation for the mechanism of action of lithium salts. - In 18 patients under long-term lithium treatment and 10 normal volunteers before and after one week of lithium application the EEG was recorded and performance as well as psychophysiological tests were carried out. The results, particularly of the d-2-test point to a considerably reduced performance in patients and normal volunteers. The EEG data in both groups suggest reduced vigilance until stage B3 or C (in the sense of Head of Bente); CFF was significantly elevated only during the first days of lithium application in the normal volunteers. Whereas EEG changes persist, other symptoms as CFF reduction, fine hand tremor, or reduced performance resolved at least partially 7 days after lithium withdrawal in the normal volunteers.