Single 30- mg doses of chlordiazepoxide HCl were administered to six healthy human subjects by the intravenous, oral, and intramuscular routes. Plasma concentration- time curves following intravenous administration were satisfactorily described by a biexponential equation consistent with a two-compartment open model system. Mean values of half-lives for the so-called distribution and elimination phases were 0.252 and 9.39 hr, respectively. The mean values for the volume of the central compartment (V ₁) and volume of distribution (V_db )(V_{d_\beta } ) were 18.0 and 30.9% of body weight, respectively. Following oral administration, the drug was rapidly and completely absorbed. Absorption was first order (t_1/227 min), and three of the six subjects showed a discernible lag time of approximately 20 min. Drug absorption following intramuscular administration was comparatively slow. A two- compartment muscle model comprised of precipitated and solubilized drug in the muscle was found to satisfactorily characterize the absorption process following administration by this route.