This experiment investigated whether (-)-epigallocatechin-3-O-gallate (EGCG) (5-20 mg/kg, p.o.) has hypnotic effects and/or enhances pentobarbital-induced sleeping behaviors and whether these effects are mediated by γ-aminobutyric acid (GABA) receptors. EGCG prolonged sleeping time induced by pentobarbital (42 mg/kg, i.p.) and reduced sleeping latency induced by pentobarbital similarly to muscimol (0.2 mg/kg, i.p.), a GABA(A) receptor agonist in mice. EGCG also increased sleeping rate and sleeping time when co-administered with pentobarbital (28 mg/kg, i.p.) at a subhypnotic dosage. In addition, EGCG and pentobarbital increased chloride (Cl(-)) influx in primary cultured cerebellar cells. EGCG and pentobarbital decreased GABA(A) receptors α-subunit expression and had no effect on the expression of β- and γ-subunits and of glutamic acid decarboxylase in the hippocampus of rats. In conclusion, the EGCG enhancement of Cl(-) influx may play an important role in pentobarbital-induced sleeping behaviors.