The claim that the hearts of animals chronically exposed to DDT are unduly sensitive to the cardiotoxic effects of vasopressin was reexamined. Rats and rabbits were fed a diet containing DDT for 8 months and were given weekly doses of vasopressin, which causes a temporary myocardial ischaemia. Electrocardiograms, recorded at 2-week intervals, showed no significant increase in the incidence of cardiac arrhythmias in the DDT-fed animals. Intravenous noradrenaline given at the end of the 8-month period did not produce a greater incidence of arrhythmias in the DDT-fed animals than in the controls. Isolated heart muscle preparations from the DDT-fed and control rats did not differ in their peak developed tensions and maximum rates of tension development. DDT, TDE, and DDE levels in heart muscle and fat were below detectable levels in the control rats, whereas in the DDT-fed rats they were at least 100 times those normally found in human tissues.