A test procedure, inducing a defined state of vigilance in rats, has been investigated to ascertain its usefulness for EEG monitoring of the effects of drugs. In order to prevent spontaneous fluctuations in the level of vigilance, rats were trained to walk in a slowly rotating treadwheel. The level of vigilance recorded under these conditions could be shifted by amphetamine, 1.0 mg/kg p.o., to a higher state of arousal, as shown by a reduction in power in all frequency bands, and by diazepam, 0.3 mg/kg p.o., to a lower state of arousal, as indicated by a rise in the activity at low frequencies. Imipramine (1.0, 3.0 and 10.0 mg/kg p.o.) caused a dose-related reduction in power in the intermediate frequency bands (3–18 Hz).