The pharmacokinetics of a fixed dose of midazolam (0.3 mg kg-1 i.v.) were studied in detail in 115 healthy patients or volunteers and nine were found with a prolonged elimination half-life. A further 102 patients had an abbreviated pharmacokinetic study, of whom five showed a similar abnormality. Defective hepatic metabolism of midazolam may be a factor in the aetiology of what appears to be a true phenomenon, occurring in 6% of over 200 fit subjects given a standard dose of the drug.