The hypnotic activity of N³-phenacyluridine in 2.0μmol/mouse by intracerebroventricular (i.c.v.) injection was 20 times stronger than that of known N³-benzyluridine. In 0.5μmol/mouse, i.c.v., this compound strongly potentiated both pentobarbital- and diazepam-induced sleep as compared to N³-substituted uridines, including N³-benzyluridine. Furthermore, the compound caused motor incoordination as well as decreasing spontaneous activity in the same dose. These results indicate that among the N³-substituted uridines and related compounds previously reported, N³-phenacyluridine possesses potent depressant effects.