6R-L-erythro-5,6,7,8-tetrahydrobiopterin (6R-BH4) is a coenzyme for tyro-sine, tryptophan and phenylalanine hydroxylases. Male C57BL/6J and DBA/ 2J mice were given 6R-BH4 (0,12.5, 25.0 and 50.0 mg/kg of body weight, i.v.) or saline, and then 4 h later, all animals were injected with ethanol (EtOH) (4.0 g/kg, i.p.), which causes them to lose the righting reflex, to investigate the differences in EtOH-induced sleeping time. 6R-BH4 pretreatment reduced EtOH-induced sleep time in DBA/2J mice with lower alcohol preference (p < 0.05), which showed no changes in the pharmacokinetics of blood EtOH. No changes in EtOH-induced sleep time were observed in C57BL/6J mice with higher alcohol preference. These results indicate that the sensitivity to EtOH in mice with lower alcohol preference was associated with 6R-BH4 activity in the CNS.