Sensitivity of the central nervous system (CNS) to the hypnotic effect of hexobarbital was assessed by analytical, pharmacokinetic, and electroencephalographic techniques in male and female rats and in mice of both sexes by the first two techniques. These experimentally and conceptually diverse measures yielded strikingly similar estimates of CNS sensitivity to hexobarbital which was independent of both species and sex, while well known species and sex (rat) differences in duration of response to hexobarbital were demonstrated.