Established hypertension is associated with enhanced susceptibility to life-threatening arrhythmias. However, little is known about the effects of the development of hypertension on the electro-physiological properties of the heart. To study this relationship, dogs were chronically instrumented for recording mean arterial pressure and conducting cardiac electrical testing during right ventricular pacing using an intracavitary bipolar catheter. In normotension, ventricular fibrillation threshold was determined under alpha-chloralose anesthesia, after which perinephritic hypertension was induced by wrapping one kidney in silk followed by contralateral nephrectomy. During serial testing of the same animals, ventricular fibrillation threshold was significantly increased early in the development of hypertension (2 to 3 weeks after renal wrapping), but found to be significantly reduced at 6 to 7 weeks after renal wrapping. The increase in ventricular fibrillation threshold was eliminated by cholinergic blockade.