A double-blind, randomized, cross-over study has been conducted with zopiclone (Imovane), a new cyclopyrrolone hypnotic in ten healthy volunteers. Hypnotic efficacy has previously been demonstrated at a dose of 7.5 mg. The present study was designed to determine the overnight residual effects of different doses of the drug ranging from 2.5 to 10 mg. Measurement of complex reaction time was used as an objective test of impairment of psychomotor function. To supplement this, the volunteers were asked to report, by means of visual analogue scales, any perceived changes in their performance in the complex reaction time test, in their mood, and in the onset and quality of sleep. Complex reaction time testing revealed no significant impairment except at 12 h after the 10 mg dose. Subjective assessments of onset of sleep showed a dose-dependent shortening of sleep latency, confirming the hypnotic action of the drug. Volunteers were aware of a decline in psychomotor performance, of some sleepiness on awakening, and decreased alertness after previous night doses of 7.5 and 10 mg. It is concluded that a dose of 7.5 mg is optimal, producing significant hypnotic effect with minimal residual impairment.