This investigation compares the effects of single and double doses of nitrazepam (5 and 10 mg) and zopiclone (7.5 and 15 mg) and placebo for 1 night in 40 psychiatric patients. The results indicate that zopiclone is an active hypnotic compound, comparable in its effects to those of nitrazepam, the higher dosage being best adapted to the type of patients included in the study.