In order to elucidate the mechanism(s) behind the interactions between barbiturates and Ca(2+) antagonists, the effects of three structurally diverse types of Ca(2+) antagonists combined or not with 5-HT on pentobarbital-induced hypnosis in mice were investigated. The results showed that dihydropyridine derivative nifedipine (10.0 and 20.0 mg/kg, p.o.) and other types of Ca(2+) antagonist, verapamil (5.0 and 10.0 mg/kg, p.o.) and diltiazem (2.5, 5.0 and 10.0 mg/kg, p.o.) increased both the sleeping time in hypnotic dosage of pentobarbital (45 mg/kg, i.p.) treated mice and the rate of sleep onset in the sub-hypnotic dosage of pentobarbital (28 mg/kg, i.p.) treated mice in a dose-dependent manner, respectively, and these effects were significantly augmented by 5-hydroxytryptophan (5-HTP), the immediate precursor of 5-hydroxytryptamine (5-HT). Pretreatment with p-chlorophenylalanine (PCPA, 300 mg/kg, s.c.), an inhibitor of tryptophan hydroxylase, significantly decreased pentobarbital-induced sleeping time and nifedipine (10.0 mg/kg, p.o.), verapamil (5.0 mg/kg, p.o.) and diltiazem (2.5 mg/kg, p.o.) abolished this effect. From these results, it should be presumed that the augmentative effect of L-type Ca(2+) channel blockers on pentobarbital-induced sleep may be influenced by serotonergic system.