Barbiturate treatment can enhance drug oxidation in man, but its effects on glucuronidation of clinically used drugs are unknown. We have studied the effect of 10 days treatment with pentobarbital on oxazepam glucuronidation in six volunteers. The diastereomeric (+ and -) glucuronides in urine were analyzed separately by high performance liquid chromatography and oxazepam in plasma by gas liquid chromatography. The area under the plasma concentration time curve for oxazepam (15 mg sod) decreased after pentobarbital treatment in all subjects, mean 10.4 before and 6.9 h X mumol/l after treatment (P less than 0.05) and oxazepam's plasma clearance increased by about 50 per cent. The glucuronides appeared more rapidly in the initial urinary fractions. The ratio between the glucuronides (+/-) appearing in urine decreased on pentobarbital treatment. Our data indicate that pentobarbital treatment increases the clearance of oxazepam by increased glucuronide conjugation, probably reflecting barbiturate induced changes of UDP glucuronyltransferase.