Hypnotic effects following administration of three doses of pentobarbital were evaluated in mice selectively-bred for differential hypnotic sensitivity to ethanol. Although the ethanol-sensitive Long-Sleep (LS) line displays greater sedation to a wide variety of CNS depressants (alcohols, barbiturates, benzodiazepines, general anesthetics), when compared to the ethanol-insensitive Short-Sleep (SS) line, the response pattern to pentobarbital remains equivocal. Thus, to clarify the effect of pentobarbital, certain variables (dose, sex, circadian rhythmicity) believed to be important in the expression of sleep time were evaluated. For all doses examined "sex" and "time of day tested" impacted on sleep time. With these provisos, 40 mg/kg consistently induced shorter sleep time in SS mice. The 60 mg/kg dose either failed to distinguish these two lines, or induced greater sleep times in the SS mice. The 80 mg/kg dose tended to have the same effect as the 60 mg/kg dose, but to a greater degree. Overall, it appears that for each line the dose response curve for pentobarbital is sigmoidal, but that the slope of the curve for the middle range of doses is greater for the SS line. Since pentobarbital has a unique effect on these lines of mice that is dissimilar to those reported for other barbiturates, the implication is that an additional factor, that is unimportant for other barbiturates, is essential for pentobarbital-induced hypnosis. Factors that could be responsible for this effect include differential metabolism of Gabaergic receptor dynamics.