The levels of gentamicin in blood and in secretions of the tracheobronchial tree were measured in 14 children (8 with cystic fibrosis and 6 with tracheostomy) in a cross over fashion after the administration of 40 mg of gentamicin by aerosol or by endotracheal injection. High levels of gentamicin (greater than 20 mug/ml) within the bronchial secretions were observed in 7 children after aerosolization and in 11 children after endotracheal instillation. Corresponding blood levels were low (less than 3 mug/ml) in all patients and no detectable levels were found in 10 children after aerosolization and in 6 children after endotracheal instillation. No significant differences were observed between children with cystic fibrosis and those with tracheostomy. Since intramuscular injection of gentamicin (a single dose of 1.5 mg/kg) resulted in low levels of gentamicin within the bronchial secretions (less than 2 mug/ml in 10 patients, among whom 4 had undetectable levels); it is concluded that the administration of an antibiotic such as gentamicin, directly to the trachea by endotracheal injection or by aerosolization might prove to be helpful when the infection is confined mainly to the tracheo-bronchial tree.