Myocardial lesions induced by restraint stress were studied by histochemical and electron microscopical methods in crossbred pigs of Yorkshire and Swedish Landrace. Dehydrogenases, cytochrome oxidase and unspecific esterase were applied to fresh-frozen sections of heart muscle. The damaged muscle fibers exhibited an initial increase in formazan deposits and then a decrease and total loss of enzymatic activity. A PAS-positive amylase-resistant material was found in the damaged muscle fibers. Ultrastructurally, accumulation of lipids and a reduction or total loss of glycogen were demonstrated in the damaged myocardium. Myofibrils were contracted in the periphery of the necrotic foci. Within the degenerating and necrotic areas the changes ranged from clumping to complete lysis of the myofilaments. The mitochondria were mostly swollen, with focal loss of the cristae. They often exhibited electron-dense bodies of different appearances. Swelling of the SR and T system was observed, as well as inflammatory cells with dilated cisterns of rough-surfaced endoplasmic reticulum. The myocardial lesions were similar to those observed in rats after large doses of catecholamines were administered. It was concluded that the myocardial lesions were caused by liberated catecholamines.