Previous studies from this laboratory have demonstrated early and progressive alterations in the ST-T period of the fetal and neonatal electrocardiogram in relation to asphyxia. The aims of the present study were to investigate the metabolic background of these hypoxic ECG changes by means of serial myocardial biopsies in fetal lambs, relating these changes to the hypoxic depletion of glycogen, ATP and creatine phosphate stores in the heart and to the altered myocardial performance as measured by heart rate, mean arterial blood pressure, combined cardiac output and max. dP/dt. The experiments were performed on 21 fetal lambs, acutely exteriorized and subjected to graded hypoxia. During hypoxia there was a significant relationship between the degree of changes in the ST-T period according to a scoring system and the depletion of myocardial glycogen and ATP, a highly significant correlation between the rate of myocardial glycogenolysis and the rate of increase in T wave amplitude, and a parallelism between the amount of glycogen available and fetal cardiovascular function. The myocardium was capable of regenerating its glycogen stores under conditions of adequate oxygenation and in the absence of acidosis and hypoglycaemia.