The scanning electron microscope had been applied to the study of cellular alterations which are caused by myocardial ischemia. This instrument allows rapid tissue preparation and, while resolution is less than can be obtained with the transmission electron microscope, it provides a somewhat three-dimensional appearance of the cell's ultrastructure. The early alterations consist mainly of swelling of organelles, separation of fibrils, and clumping of nuclear chromatin. Within 5 hr disruption of the T-tubules and sarcoplasmic reticulum occurs, nuclear distortion is frequent, and fibrillar destruction is commonly seen but is often focal. Extensive fibrillar alterations occur after 10 hr and become more exaggerated at 24 hr after the onset of ischemia. However, even after 24 hr many mitochondria and nuclei, though damaged, are recognizable. This study suggests that scanning electron microscopy may play a useful role in the study of pathological alterations at the ultrastructural level.