The goal of the present study was to classify psychotropic drugs on the basis of EEG-defined rat sleep-waking behaviour. Using an automated sleep classification system it was found that some of the drug-induced changes in sleep-waking behaviour were specific for the pharmacotherapeutic treatment class to which the drug belonged. In several preliminary experiments we further found that drugs may have effects on rat EEG independent of their effects on rat sleep-waking behaviour and that these pharmaco-EEG effects may be different for the various sleep and waking stages. By analysing sleep class-independent EEG-spectral parameters a single drug effect score can, moreover, be obtained giving information on drug pharmacodynamics. The drug-induced changes in sleep-waking behaviour were used to classify a large number of drugs into several therapy classes by means of a discriminant analysis procedure. Antidepressants, antipsychotics and stimulants were discriminated successfully from each other and from placebo by this system, whereas nootropics classified as placebo. Anxiolytics, hypnotics and anticonvulsants classified poorly. Their classification is hampered by the lack of specific compounds. Assigned drug class and assignment probability were dose dependent. In the discussion of the present study it is suggested that animal pharmaco-sleep and pharmaco-EEG studies are not mutually exclusive approaches, but that they may complement each other.