Chronic implantation of four bipolar concentric electrodes into frontal cortex, hippocampus, striatum and reticular formation of the rat allows continuous recording of bioelectric potentials during the action of various drugs. Frequency analysis of the potentials serves to quantify EEG changes over longer periods of time. Segmentation of the spectra into six frequency bands and integration of their power provides parameters by which the different drugs can be differentiated from each other. The action of classic tricyclic antidepressants like amitriptyline and imipramine as well as the effect of doxepine is characterized by a general decrease in power with respect to all frequency bands and all brain areas. Amitriptylinoxide can be distinguished from them by its lack of decrease in beta-2 power and a smaller decrease in alpha-1 power. Amphetamine lacks decreases in alpha-1 frequencies in the striatum and the reticular formation. Both diazepam and haloperidol show increases in beta-2 power; haloperidol increases alpha-1 power, whereas diazepam diminishes it. LSD can be differentiated from amphetamine by its increases in alpha-1 power in the hippocampus and striatum. Thus all antidepressants show very similar changes with respect to the frequency patterns obtained after drug injection, whereas drugs used for other indications can be well distinguished from each other and also from antidepressants.