This study includes 30 children below the age of 3 years, distributed in three groups of 10 children each. One group received 15 mg/kg/day of rifampicin (RAMP), the other received 15 mg/kg/day of isoniazid (INH) and the third received both drugs (RAMP + INH) at the same doses. Plasma levels of RAMP and INH were determined. At the same time, some hepatic function parameters were analyzed: bilirubin, transaminases, alkaline phosphatases and prothrombin. Plasma levels of INH administered alone show significantly higher levels on the 7th day with respect to the 1st day, thus demonstrating a marked tendency towards the accumulation of successive doses. Very significant differences exist between initial and final values of transaminases in the RAMP + INH group. There are no significant differences in the other two groups. These data suggest a hepatic overload when both drugs are simultaneously administered.