The disposition of hexobarbital was studied in rats after i.v. and i.a. administration. In addition to sleeping times, plasma concentration profiles were measured. No significant differences were found in sleeping times, volumes of distribution, elimination half-lives or systemic clearances between these different routes. The average elimination half-lives were 13.5 +/- 0.8 (n = 17) and 11.6 +/- 1.9 min (n = 21) (means +/- S.E.M.), respectively, whereas the systemic whole blood clearance values were 75.4 +/- 3.4 (n = 17) and 85.8 +/- 3.5 ml/min/kg (n = 21) (means +/- S.E.M.). The values of the latter parameter approach hepatic blood flow in the rat (i.e., 100 ml/min/kg) and therefore the oral availability of hexobarbital was established by comparing areas under plasma concentration time curves, after i.v. and oral administration to the same rat. Oral availability was found to be only 36%, which corresponds to an extraction ratio of 64%. The consequences of such a "first-pass" effect are discussed in view of the use of hexobarbital as a model substrate for measuring drug-metabolizing enzyme activity. Furthermore, it was found that anesthesia as induced by diethylether during the experiments resulted in a very significant inhibition of the rate of hepatic metabolism of hexobarbital; the elimination half-life increased by about 50% due to a similar decrease in the systemic clearance. The protein binding of hexobarbital in rat plasma amounted to 51.4 +/- 1.2% (mean +/- S.E.M., n = 15) and it was found not to be dependent on the plasma concentration of hexobarbital in the range encountered in vivo.