The usefulness of radioreceptor assay (RRA) in the pharmacokinetic studies on midazolam - the first benzodiazepine with water-soluble salts - was compared with that of gas liquid chromatography (GLC) in 18 patients operated on under local anesthesia in supine position. There were no significant differences observed in the serum levels determined with these two methods after a single i.v. injection of midazolam 0.075 mg/kg although considerable variability was manifest. Similarly, the pharmacokinetic parameters did not differ from each other in view of the concentrations determined by RRA or GLC. The pharmacokinetics of midazolam were compared in three patient groups on the basis of the GLC determined concentrations: a) in pregnant patients operated on at term under epidural analgesia (group 1, n = 6), b) in nonpregnant women operated on under epidural analgesia (group 2, n = 6), and c) in 3 male and female subjects operated on under brachial plexus blockade (group 3). The total serum clearance, the most important pharmacokinetic parameter, was highest in group 1 patients and lowest in group 3. However, these significant differences were not reflected so clearly in the elimination half-life of midazolam. There were no differences in tissue distribution (Vd alpha, Vd beta) between the three patient groups. A highly significant correlation was observed between the serum concentrations and sedative effects of midazolam. Midazolam is a short-acting sedative-anxiolytic agent with a rapid onset of action especially useful for patients operated on under local anesthesia. Apparently, the pharmacokinetic differences observed in this study are of no great clinical significance in single dose administration.