Histoenzymological techniques were used to examine ATPase activity in rat heart muscle fibres after experimental infarction. 25 hours after coronary ligation, ATPase activity in all ventricular section fibres was high, homogeneous at pH 9.4, sections. 48 hours after ligation, necrotic ventricular fibres appeared, leaving only a thin layer of fibres which had apparently preserved their myofibrillar ATPase. These results indicate that, unlike mitochondrial enzyme activity, myofibrillar ATPase activity is relatively resistant to ischaemia.