To investigate the pharmacological effects of zopiclone in the central nervous system, we monitored event-related potentials (ERPs, N100 and P300), the reaction time and the scores for subjective mental and emotional state (by using the Polarity Profile Questionnaire, PPQ) on 10 healthy right-handed male volunteers between 21 and 32 years old. Subjects were randomly allocated to receive a single oral administration of zopiclone 7.5 mg or placebo at the first-session, and the drug was crossed over at the second session 1 week later. ERPs, the reaction time and the psychometric scores were monitored at pre-drug, and 1, 3 and 5 hours after drug administration. In the zopiclone group, the reaction time was prolonged significantly at 1 hour after drug administration, the amplitudes of the ERPs significantly decreased at 1 and 3 hours after, and the PPQ scores showed significant changes. Maximum effects on the psychometric scores appeared later than those on ERPs and reaction time. Zopiclone decreased the arousal level and sensory sensitivity, and induced sedation through changes in subjective mental and emotional states.