The single oral dose kinetics of carbamazepine-10,11-epoxide (CBZ-E), the active metabolite of carbamazepine, were studied in six epileptic patients, stabilized on phenobarbital (PB) monotherapy, and in six drug-free health volunteers. The epoxide metabolite was administered as an enteric-coated tablet at the dose of 200 mg to the patients and at the dose of 100 mg to the volunteers. Patients had a significantly higher plasma clearance of CBZ-E than the control group (mean values +/- SD = 220.2 +/- 63.5 versus 112.5 +/- 46.0 ml/h/kg, p less than 0.007) and a significantly shorter plasma half-life (mean values +/- SD = 4.3 +/- 1.0 versus 6.7 +/- 0.8 h, p less than 0.0015). These results suggest that PB induces CBZ-E metabolism.