We studied the structural alterations that follow the application of currents similar to those used in clinical electric countershock procedures in cultured chick myocardial cells. Myocardial cells 48 hours in culture were subjected to electric field stimulation. The cells then were fixed for electron-microscope examination 30 seconds postshock. The ultrastructural alterations that appeared after an 80 V/cm shock included the appearance of >25% of the mitochondria per cell in the "condensed" configuration and mitochondria with swirled or tubular cristae. Large-amplitude swelling of the mitochondria and swollen endoplasmic reticulum also were observed in a few cells. The alterations that appeared after a 200 V/cm shock included contracture and disorganization of the myofibrils, large-amplitude swelling and loss of cristae in the mitochondria, swollen endoplasmic reticulum and nuclear envelope, and intracellular edema. The degree of ultrastructural change was dependent on the intensity of the shock. These structural alterations are consistent with an osmotic imbalance which occurs during a shock-induced prolonged depolarization of the cell membrane (Jones et al., 1978a), possibly due to a transient dielectric breakdown during the shock, and may underlie the arrhythmias and necrosis which often occur after high-intensity clinical and experimental eountershock procedures in vivo. Circ Res 46: 387-394, 1980