The development of the benzodiazepin molecule has led to drugs which are specially indicated in the management of anxiety. Such drugs may be ingested overnight in which a beneficial effect on sleep is followed by an anxiolytic effect the next day, or during the day. In each case it is desirable that their day time effect is without or with only minimal impairment of performance. In the present paper studies on the effects of two anxiolytic (potassium chlorazepate and clobazam) on sleep and on performance will be reviewed. Potassium chlorazepate is usually given as a single dose (15 mg) overnight. It has useful hypnotic activity and a sustained anxiolytic effect the next day related to the activity of its long-acting metabolite, nordiazepam. Further, unlike most if not all other, 1,4 benzodiazepines it is difficult to unequivocally impairment of performance. On the other hand clobazam is usually given as repeated doses (10-20 mg) during the day, and with this 1,5-benzodiazepine it is also difficult to establish impairments of performance. Anxiolytics without impairment of performance during the day have obvious clinical advantages, and if they also have a useful hypnotic activity they can be particularly appropriate for the management of insomnia secondary to anxiety.