The plasma catecholamine and serum cortisol responses to cardiac arrest (ventricular fibrillation), cardiopulmonary resuscitation (CPR), and ventricular defibrillation were examined in 10 intact (sham-operated controls) and 10 bilaterally adrenalectomized dogs. One hour after surgery, the cardiac ventricles were electrically fibrillated, and 30 s later Standard American Heart Association CPR was begun. After 12 min of CPR, the ventricles were defibrillated. Cardiac arrest per se results in a massive increase in plasma epinephrine and norepinephrine concentrations and indicates that the adrenal medullas are the predominant source of this response. Although the epinephrine response was virtually nonexistent in the adrenalectomized dogs, the norepinephrine response was approximately 30% of that in the sham-operated control animals. Thus there is an adrenomedullary, and perhaps a sympathetic neural, component to the sympathochromaffin response to cardiac arrest. Resuscitation from experimental cardiac arrest tended (P greater than 0.05 less than 0.1) to be lower in the adrenalectomized dogs (1 of 10) than in the animals with intact adrenal glands (6 of 10).