This study was undertaken to sort out a safer or better contrast agent when the agent was introduced endobronchially either purposely or inadvertently during the gastrointestinal examination. For this purpose, radiographic and histopathologic evaluations were carried out in rats after endobroanchial instillation of Gastrografind, barium sulfate, Hytrast and Isovist-300. The results were as follows. Anatomic definition of the tracheobronchial tree was the best with barium sulfate and Hytrast, follwed by Isovist-300 and gastrografin. The greatest acute inflammatory effects and pulmonary edema were evoked by Gastrografin. The least tissue reactionswere observed with Isovist-300. In cases with high risk of aspiration such as tracheoesophageal fistula, children and postoperative state, nonionic agents such as Isovist-300 should be considered as a gastrointestinal diagnostic agent.