In rats and rabbits with chronically implanted electrodes CS-430(10-bromo-2, 3, 7, 11b-tetrahydro--11b-(2-fluorophenyl)-oxazolo(3, 2d) [1, 4]benzodiazepine-6(5H)-one) (3 mg/kg, .o.) decreased the slightly awake period and increased the slow-wave sleep period. The onset time of sleep was significantly shortened. In the rat pre-treated with p-chlorophenylalaine (500 mg/kg, p.o.), the awake period was remarkable increased and the slow wave sleep period was considerably decreased at 48 hours after the pre-treatment. In these rats CS-430 given in the same dose decreased significantly the awake peiod and the onset time of sleep and increased the slow wave sleep period. CS-430 raised the threshold of the arousal response with stimulation of the posterior hypothalamus, but not of the mesencephalic reticular formation. These effects of CS-430 were almost as potent as those of nitrazepam (NZP). CS-430 reduced duration of after-discharges following stimulation of the hippocampus, depressed the decerebrate rigidity and inhibited activity of lumbar gamma-motoneurons in anesthetized cats. CS-430 (10 mg/kg, p.o.) inhibited the evoked responses recorded from the proreus gyrus following stimulation of either the baso-magnocellularis of the amygdaloid nuclear or the ventral hippocampus, although such did not inhibit the responses between the olfactory bulb and the pyriform cortex, the thalamus and the sensory-motor cortex. Thus CS-430 has a sleep-inducing effect in animals of almost the same potency as is seen with NZP, and site(s) of the action in the limbic system appear to be specific.