It is well known that psychotropic drugs can induce EEG alterations. Dose dependence seems established; however, there are no data concerning the impact of plasma levels. The authors investigated the influence of clozapine plasma levels on the frequency of EEG alterations. Data from 29 inpatients (18 male, 11 female, 31.7±10.2 years) receiving clozapine in a dose range between 25 and 600 mg were collected prospectively. There was no psychotropic or anticholinergic comedication. All patients had normal EEGs before taking clozapine. Fifteen patients showed pathological changes (group 2) and 14 no changes (group 1). Discriminant analysis showed that EEG changes are dependent on plasma levels (P=0.0009, plasma levels in group 1 mean 81.6 ng/ml, ±SD 64.6, in group 2 235.7 ng/ml, ±169.8). A total of 72.4% of the patients were correctly classified as having either pathological EEG changes or none by this analysis. Variables such as dose, age, sex, weight and duration of treatment were not statistically relevant. It can therefore be suggested that clozapine plasma levels are a valid indicator for the appearance of electrophysiological reactions.