Circadian susceptibility of sleeping induced by pentobarbital was observrd in male DDO mouse treated with phenobarbital and ginseng saponin. The pentobarbital elimination rate was also measured in the same animal. The mouse had been maintained for one week under 12 hours of artificial illumination extending from 06:00 to 18:00 hours alternating with 12 hours of darkness. During the period the animals were administered intraperitoneally with 100mg/kg of phenobarbital for three days or 10mg/kg and 100mg/kg of ginseng saponin for seven days. At 24 hours after last injection pentobarbital sleeping time and elimination rate were measured following intraperitoneal administration of 50mg/kg of pentobarbital sodium. In a control group treated with saline, the duration of pentobarbital-induced sleep varied with circadian rhythmicity, which had a trough at 02:00 hours of light phase and a crest at 14:00 hours of dark phase. And the elimination rate measured at 02:00 hours was faster than that at 14:00 hours. Pretreatment with phenobarbital markedly shortened the pentobarbital steeping time and abolished the circadian rhythmicity. Those were correlated with the increased pentobartital elimination by phenobarbital throughout light and dark phases examined. Ginseng saponin, given for seven days in a dose of 10mg/kg or 100mg/kg, did not affect the circadian rhythmicity of sleeping and the elimination rate. Sleeping time during light phase, however, was somewhat shortened in ginseng treated animals, which was not matched with the finding of unaltered elimination rate. It seemed that the central nervous system stimulating effect of ginseng saponin might be involved in the findings observed.