Particulate iodipamide ethyl ester, a new hepatolienographic x-ray contrast agent, was intravenously injected into rats. Lung and kidney biopsies taken at various intervals after the injection were examined by light and electron microscopy. IDE particles could be found in the lung capillaries phagocytized by polymorphonuclear neutrophils (PMNs). There were also free particles in the alveolar capillaries in the samples taken 5 min to 4 hours after the injection. No aggregates or emboli were seen. Two days or more after the injection no intra- and extracellular particles were present. The PMNs underwent transient local hydropic degeneration; the lung cells were morphologically intact. In the kidneys, the particles first appeared in both cortical and medullary capillaries. No emboli were observed. The kidney cells did not ingest IDE, but polymorphonuclear neutrophils (PMNs) with ingested IDE were often seen loosely attached to the glomerular capillary walls. In addition, free particles were evident in the capillaries in the samples taken up to 1 hour after injection. All particles in subsequent kidney samples were located in PMNs in the glomeruli. After three or more days the renal tissue was totally devoid of particulate IDE. No morphological evidence of kidney cell injury was observed.