A crossover study was performed in five adult tracheotomized patients without respiratory tract disease to investigate the tracheobronchial kinetics of intramuscularly and endotracheally administered gentamicin in the absence of infection. Although intramuscular injection of 2 mg/kg of gentamicin yielded adequate levels in the serum, the concentrations in the bronchial secretions of noninfected patients were not adequate to inhibit strains of Pseudomonas aeruginosa isolated from other patients with clinical infections. Conversely, endotracheally administered gentamicin resulted in high and sustained concentrations in the bronchial secretions that were many times superior to the minimum bactericidal concentration of gentamicin against Pseudomonas aeruginosa. Gentamicin administered by aerosols resulted in concentrations within the bronchial secretions and sputum that were adequate to kill more than 90 per cent of the strains of Pseudomonas aeruginosa isolated in this hosopital. These studies suggest that endotracheally administered gentamicin might prove to be an adequate adjunct for the treatment of severe Pseudomonas infection of the tracheobronchial tract, particularly in the absence of foreight bodies and abnormalities of structure or function.