In 12 malnourished children, who were treated for tuberculosis, plasma levels of streptomycin and isoniazid were followed. Streptomycin was administered i.m. in a dose of 25-50 mg/kg/24 hours. High initial plasma levels were reached (mean: 44.3 mug/ml at 30 min). Streptomycin levels were followed for 5 hours and the mean plasma level at that time was 17.0 mug/ml. From the present data a plasma half life of streptomycin of 3.5 hours has been estimated. It is advised that streptomycin should not be given in doses above 25 mg/kg/24 hours to avoid potential toxic plasma levels especially if plasma levels cannot be measured. It is also concluded from our study that renal function is not affected in malnourished children to an extent where streptomycin clearance is greatly affected. Isoniazid was given orally, 10 mg/kg/24 hours. From 30 min to 6 hours after administration, mean plasma levels of isoniazid above 0.5 mug/ml were observed. In all children measurable plasma levels were obtained. It is concluded that also children with malnutrition can absorb isoniazid after oral administration. From our data it is suggested that the majority of the children in our study were rapid inactivators of isoniazid.