Barbituric acid (BA), allobarbital (AlloB) and seven kinds of allyl substituted barbituric acid were prepared and their pharmacological activity (hypnotic and anticonvulsant activities) was evaluated in mice. The interactions of BA and these allyl substituted barbituric acids with several barbiturates were also studied. Among them, N,5,5-triallyl-BA (6) exhibited some hypnotic and anticonvulsant activities, but the other compounds did not show any activity. BA (160 mg/kg, i.p.) was found to possess a prolonging effect on pentobarbital (PB)-induced sleep. N-Allyl-BA (1), 5-Allyl-BA (2), N,N''-,5-trially-BA (5), 6 and N,N'',5,5-tetraallyl-BA (7) (60 or 160 mg/kg, i.p.) also prolonged PB-induced sleeping time. Compound 6 was most potent on interaction with all barbiturates used, except for barbital (B). Compound 7 prolonged AlloB-, PB- and amobarbital (AB)-induced sleeping time, but not the other barbiturates-induced sleeping time. The prolonging effects of BA, 6 and 7 on PB-induced sleep were dose-dependent. Compounds 3-5 exhibited a prolonging effect on thiopental (TP)-induced sleep. Although compound 3 prolonged phenobarbital (PheB)-induced sleeping time, the compound and 4 did not show any prolonging effect on PB-induced sleep. These results indicate that the position and number of allyl group substituted play an important role on their depressant activity, and that BA itself possesses some depressant activities.