Thirty otherwise healthy patients received a 100-mg oral dose of chlordiazepoxide HCl just prior to surgical procedures using spinal anesthesia. Fourteen of these patients had also received 100 mg on the night before surgery. Simultaneous samples of venous blood and cerebrospinal fluid (CSF) were taken immediately prior to injection of spinal anesthesia and were assayed for concentrations of chlordiazepoxide (CDX) and its major metabolite, desmethylchlordiazepoxide. Plasma concentrations of CDX ranged from 2.32 to 13.34 mug/ml. Simultaneous CSF concentrations were considerably lower, ranging from 0.04 to 0.34 mug/ml. Equilibration of CDX between plasma and the lumbar sampling site appeared to be complete within 2 hr of the most recent dose. After attainment of distribution equilibrium, simultaneous plasma and CSF concentrations of CDX were hightly correlated (r = 0.76), with a mean CSF-plasma concentrations ratio of only 0.043 (range; 0.02 to 0.06). The limited passage of CDX into human CSF is probably due to extensive binding to plasma protein. Assuming that transfer of CDX from plasma to CSF is governed by passive diffusion, the extent of plasma protein binding of CDX in healthy individuals averages about 96%.