Rifampicin suspension was administered orally at a does of 34mg/kg to six rabbits after 5, 10 and 20mg/kg pretreatment of isoniazid twice daily for 9 days. The blood level of rifampicin was decreased significantly by isoniazid 10mg/kg 20mg/kg pretraetment. The renal clearance(CLr) of rifampicin was increased by isoniazid 20mg/kg and the biliary clearance(CLb) was incresed by isoniazid 10mg/kg and 20mg/kg pretreatmetn. Elimination rate constant(K) and time to reach maximum concentration(tmax) were increased by isoniazicl pretreatment. But half-life and maximum concentration(C max) were decreased. Relative bioavailability was decreased significantly by isoniazid 10mg/kg and 20mg/kg pretreatment.