Increasing importance is being given to the residual effects of hypnotics on the day following absorption; with zopiclone, a new hypnotic, we have studied vigilance 9, 12 and 15 hours after a single evening dose of 10 mg, on a double-blind basis in comparison with 10 mg of nitrazepam and a placebo. The assessment of vigilance is based on responses to self-rating questionnaires and psychometric tests. Twenty-one healthy subjects forming a homogeneous group as regards age, I.Q., and vigilance were included in this study. Each subject received the three products in random distributed order, with an interval of three months between each trial. The results of these tests reveal modifications which, particularly 9 hours after administration, reflect a modification in vigilance, appearing to be more marked with nitrazepam than with zopiclone. Thus, from the point of view of the residual effects of hypnotics on vigilance, it is possible to position the products tested in the following order: placebo -- zopiclone -- nitrazepam.