Experiments on two mouse strains with opposite reactions to emotional stress showed selectivity of the anxiolytic effect of endogenous dipeptide cycloprolylglycine. In the open field test cycloprolylglycine (0.01-0.10 mg/kg intraperitoneally) dose-dependently (1.8-2.1-fold) increased motor activity of BALB/c mice with manifest fear reaction and had no effect on C57Bl/6 mice with active behavior. The content of endogenous cycloprolylglycine in mouse brain correlated with the type of emotional stress reaction: its content in the brain of C57Bl/6 mice 1.5 times surpassed that in BALB/c mice. It is concluded that cycloprolylglycine is involved in the endogenous regulation of fear reaction.