The Wachstein-Meisel ATPase reaction can be used in formalin-fixed, postmortem material to demonstrate the microvasculature in the human myocardium. In cases of sudden or rapid cardiac death, the disappearance of enzymatic activity in capillary walls was observed earlier than other light-microscopic signs of damage. This was especially marked in the subendocardial regions. The staining of capillaries was found to be positive again in the varying stages of organization. Fibrotic scars were devoid of staining. The findings besides being of practical value in the diagnosis of early myocardial necrosis, support the theory that loss of capillary metabolic and functional integrity occurs early in ischaemia; it promotes the 'no reflow' phenomenon and may contribute towards myofibre necrosis.