Rifampicin is an indispensable drug along with isoniazid for the control of tuberculosis and is usually prescribed as the combination of rifampicin and isoniazid. This paper is attemtped to investigate the interaction of rifampicin and isoniazid. Isoniazid was administered orally at a dose of 30mg/kg of rabbits pretreated with rifampicin 7.5mg/kg, 15mg/kg, and 30mg/kg, respectively twice daily for 9 days. The results are as follows: The blood level and relative bioavailability of isoniazid were decreased significantly (p<0.05) by rifampicin at a dose of 15mg/kg and 30mg/kg. The renal clearance of total isoniazid and ratio of its metabolites to isoniazid were increased significantly (p<0.05) by rifampicin at a dose of 15mg/kg and 30mg/kg. It seemed to be due to enzyme induction by rifampicin. Elimination rate constant ($eta$) of isoniazid was increased and half life ($t_{1/2$eta}$) was decreased by rifampicin pretreatment. Dosage regimen of isoniazid after long term administration of rifampicin should be adjusted carefully.