Herbal medicines have received great attention as alternative medicines in recent years and are also referred to as a dietary supplement or health food. Ginkgo biloba extract (GBE) is one of the most popular herbal medicines. However, little is known about the metabolic interactions between GBE and clinically used drugs. This study attempted to investigate the effect of GBE on the pharmacokinetics of theophylline, a cytochrome P450 (CYP) 1A2 substrate and an important therapeutic agent with narrow therapeutic window used for the treatment of asthma. Commercial GBE (10 or 100 mg/kg, p.o.) or water (control group) was given to rats (6 rats for each group) for 5 consecutive days and on the sixth day theophylline (10 mg/kg) was administered either orally or intravenously. The results showed that pretreatment of rats with GBE resulted in an increase in the total clearance of theophylline of about 30% (GBE 10 mg/kg, P<0.05) and 70% (GBE 100 mg/kg, P<0.01) compared with the control group after intravenous administration of theophylline (10 mg/kg). After oral administration of theophylline (10 mg/kg), the AUC(0-24h) of theophylline was reduced by 40% following pretreatment with GBE (100 mg/kg, P<0.01). These results demonstrate that GBE pretreatment increased CYP1A2 metabolic activity and the clearance of theophylline in rats.