The pathologic effects of metrizamide on the lung were studied at 24 h and 48 h after its instillation in varying concentrations selectively into the right lungs of a series of immature dogs. Iso-osmolar and slightly hyperosmolar concentrations of metrizamide were found to cause mononuclear interstitial infiltration, focal areas of pneumonia and in some cases alveolar hemorrhage. Control bacterial studies showed that the reaction was aseptic.