The plasma concentration of INH and streptomycin was followed in 45 Ethiopian children with tuberculosis. The children were grouped according to their nutritional status as normal, underweight, marasm and kwashiorkor. INH was well absorbed in all nutritional groups to give therapeutically active plasma levels. When terminal half life (t1/2) of INH was calculated for individual patients there were more children in all nutritional groups with t1/2 greater than or equal to 2 hours than less than 2 hours, indicating a slow acetylation of INH. Streptomycin was well absorbed in all nutritional groups and therapeutic levels were obtained with 20 mg/kg i.m. After 30 mg/kg i.m. of streptomycin kwashiorkor children had an increased t1/2 of streptomycin indicating a decreased renal excretion of the drug in kwashiorkor. The clinical follow-up of the children indicated that serious tuberculosis could be successfully treated with INH and streptomycin in the doses used.