The sedative-hypnotic effects of a new benzodiazepine, 1-(2-hydroxyethyl)-3-hydroxy-7-chloro-1,3-dihydro-5-(o-fluorophenyl)-2H-1,4-benzodiazepin-2-one (SAS 643), were compared with those of flurazepam in mice and rats as well as in a double-blind clinical trial. It was found that SAS 643 has a potency 2--4 times greater than that of flurazepam while it is about one half less toxic than the latter drug. The results of the clinical trial confirm the greater activity of SAS 643 and indicate that the new benzodiazepine causes a significantly less amount of hangover than flurazepam.