The effects of a single dose of 8-chloro-6-(2-fluorophenyl)-1-methyl-4H-imidazo[1,5-a] [1,4]benzodiazepine (midazolam, Ro 21-3981, Dormicum), 15 mg, on both self-evaluated and all-night EEG-recorded sleep parameters were studied in 10 normal young volunteers. All self-evaluated parameters were modified under midazolam compared with placebo: the sleep onset latency was estimated to be shorter, the sleep duration longer, the sleep quality better. Furthermore, the feeling on awakening in the morning was rated better after midazolam. Yet, when compared with prestudy values, two of these parameters (sleep onset latency and sleep duration) were altered under placebo, despite an adaptation night, but unchanged under midazolam. All-night recordings showed a decrease in intra-sleep awakenings, an increase in stage II and III-IV and a decrease in stage I, mainly during the first sleep cycle whereas REM-sleep was not modified. No sleep latency was significantly modified. The correspondence between self-evaluated and measured sleep-duration was different under placebo and under midazolam, suggesting that an additional effect of the drug might be to modify the subjects' perception of their own sleep.