Quazepam is a new benzodiazepine that may provide good hypnotic action with negligible effect on motor coordination or respiration. Sleep laboratory studies on human volunteers have shown quazepam 15 mg to be an effective hypnotic dose, with the 30-mg dose being optimal. At these doses, there was no deterioration of motor performance, and the drug, when given nightly for two weeks, continued to exert hypnotic effects without serious adverse effects. Therefore, this study was designed to compare the respiratory effects of quazepam 15 and 30 mg to those of pentobarbital 50 and 150 mg and to placebo. Five adult male volunteers received each dose at separate times. A double-blind technique was employed for controlled rebreathing studies, to a ventilation of 40 L/min or a PETCO2 of 8%. Respiratory curves were compared with controls. The mean displacement curve at the 20-liter intercept showed a depressant effect for pentobarbital 150 mg at two hours and a stimulant effect for quazepam 15 mg at two hours but a slight depression effect for quazepam 30 mg at three hours compared with placebo. The slope of the respiratory curve was not affected.