In order to test the effect of arterial hypertension on cardiac electrical activity, isolated Langendorff perfused hearts from spontaneously hypertensive (SHR) and normotensive (WKY) rats were studied. The incidence of spontaneous ventricular arrhythmias occurring during the control perfusion was 0% (n = 28) in WKY, 31% in SHR (n = 29, p less than 0.01), 7% (n = 14) in 3-month-old SHR, and 53% in 14-month-old SHR (n = 15, p less than 0.05). The incidence of ventricular arrhythmias induced by programmed electrical stimulation (PES = stimulus train + two extrastimuli) was 18% in WKY (n = 28), 48% in SHR (n = 27, p less than 0.05), 29% (n = 14) in 3-month-old SHR, and 69% (n = 13) in 14-month-old SHR (p less than 0.05). The incidence of PES-induced irreversible ventricular fibrillation was 0% in WKY and in 3-month-old SHR (n = 42), whereas it was 38% (n = 13) in 14-month-old SHR (p less than 0.001). Myocardial norepinephrine was significantly reduced in SHR with respect to WKY, but no significant difference was observed between 3-month-old SHR and 14-month-old SHR. Thus, no correlation between myocardial norepinephrine and ventricular arrhythmias could be found. It was concluded that the duration of hypertension was the most important factor in the development of severe ventricular arrhythmias.