The scanning electron microscope was used to study the ultrastructural alterations which occur following periods of myocardial ischemia of 30 minutes and 1, 2, 5, 10, and 24 hours. The alterations were compared with those seen with the transmission electron microscope. The early changes consist of swelling of organelles and clumping of nuclear chromatin. After 5 hours, disintegration of the transverse tubular system becomes apparent, and breakdown of the connections between the nucleus and the sarcoplasmic reticulum is seen coincident with distortion of the shape of the nucleus. It is suggested that the sarcoplasmic reticulum may be a part of the "endoskeleton" of the cell and may play a role in maintaining the shape and location of the nucleus. After 10 hours, extensive membrane alterations have occurred in many of the intracellular organelles. It is concluded that the changes seen with scanning electron microscopy are consistent with those seen with transmission electron microscopy.