Dogs were used as a model for the study of the rate of appearance, peak concentrations, and rate of clearance of gentamicin sulfate in bronchial secretions. Gentamicin (dose, 1.7 mg/kg) was given by rapid intravenous infusion, by intramuscular injection, or as two divided intramuscular injections with a 4-hr interval. Intravenous infusion resulted in the highest bronchial concentrations of gentamicin, but the drug was cleared from the respiratory secretions in approximately 3 hr. In contrast, intramuscular injections gave low but more sustained bronchial leves of the drug. The time during which bronchial gentamicin concentrations exceeded a particular minimal inhibitory concentration for a susceptible bacterium such as Pseudomonas was related to the method of drug administration. Thus, to achieve the maximal antibiotic effectiveness, the minimal inhibitory concentraiton of gentamicin for the microorganism should be considered when the dose or frequency of parenteral gentamicin is prescribed.