Necrotic lesions caused by the intraductal injection of sodium taurocholate in rat pancreas were studied by electron microscopy. The early (1 and 15 min after the injection) ductal cells were often necrotic. The acinar cell damage was characterized by mitochondrial swelling, vesiculation of the granular endoplasmic reticulum, and dissolution of cellular membranes. At later time intervals (1, 3, and 6 h) there were necrotic acinar cells with heterogeneous cytoplasmic inclusions representing degenerative cell organelles and their remnants. At 6, 12, and 24 h there were autophagic vacuoles in sublethally damaged acinar cells. The zymogen granules seemed relatively well preserved throughout the experiment. The vascular lesions consisted of endothelial detachment, extravasation of erythrocytes, thrombosis, and poikilocytosis. It was concluded that the initial cell injury was caused by the detergent action of the injected bile salt.