Ischaemic contracture of the heart ('stone heart') – a condition in which the myocardium becomes irreversibly contracted – was observed in eight of 47 dogs following death from asphyxia. This phenomenon did not occur in hearts ischaemic for less than 30 min, but was seen in six of eight hearts anoxic for more than 60 min. In five of these animals serum electrolyte estimations suggested an unusual imbalance of potassium and calcium ion exchange. No other significant features were observed to differentiate this group from hearts which remained flaccid following ischaemic cardiac arrest. The possible association of this condition with a disturbance of calcium ion exchange is discussed in the light of other observations.