This experiment was performed to investigate whether 50% ethanol extracts of the combined-preparation of Longanae Arilus, Chrysanthemi Flos, Zizyphi Fructus and Ginseng Radix alba (CPE) has hypnotic effects and/or enhances pentobarbital-induced sleeping time. Locomotor activity was evaluated using a ambulometer of tilting-type. The sedative-hypnotic effects were evaluated by measuring the sleeping onset time and sleeping time in pentobarbital-treated mice 30 min. after oral administration of CPE and muscimol. The intracellular Cl- concentration of cerebellar granule cells was estimated using Cl- sensitive fluorescence probe N-(ethoxycarbonylmethyl)-6-methoxyquinolinium (MQAE). CPE (150 mg/kg) decreased the locomotor activity, but CPE itself did not induce sleep. However, CPE reduced sleeping onset and prolonged sleeping time induced by pentobarbital (42 mg/kg). In addition, CPE (2 μg/ml) and pentobarbital (2.5 μM) itself did not affect on the chloride influx in primary cultured cerebellar granule cells, but the combination of CPE and pentobarbital (2.5 μM) increased the chloride influx onto the cells. In conclusion, it is suggested that CPE might augment pentobarbital-induced sleep through the increase of chloride influx.