Three healthy volunteers (2 male and one female) participated in single- and multiple-dose pharmacokinetic studies of oral chlordiazepoxide (CDX) hydrochloride. Following single 50-mg oral doses of CDX.HCl, absorption and elimination proceeded as apparent first-order processes. Values of absorption half-life were: 14.5, 189, and 18.9 minutes; elimination half-lives were: 7.6, 9.8, and 12.6 hours. Disappearance of CDX was mirrored by appearance of its first active metabolite, desmethylchlordiazepoxide (DMCDX). During once-daily ingestion of 50 mg of CDX.HCl, observed values of CDX accumulation half-life (0.0, 5.8, and 52.5 hours) differed substantially from values predicted based upon the single-dose study; pre-dose steady-state CDX blood concentrations also differed from the predicted values. Accumulation half-lives for the metabolite DMCDX were: 17.7, 9.9, and 15.8 hours. Accumulation in blood of a second active metabolite, demoxepam (DMX), proceeded with half-life values of 21.1, 34.2, and 78.5 hours. Minimum steady-state concentrations of DMCDX and DMX exceeded those of the parent compound. Thus accumulation and persistence of at least two active metabolites during long-term treatment with chlordiazepoxide renders the drug suitable for single-daily dose therapy of anxiety.