The effect of bilateral carotid sinus denervation (CSD) on the ECG and on the monophasic ventricular action potential (MAP) was studied in 28 and 4 rats, respectively. After CSD, T wave changes, similar to those seen in man after carotid endarterectomy, were observed and mean Q-T prolongations of 19 ms were recorded (P less than 0.001). Mean MAP increased by 10 ms (P less than 0.02). P-QRS and heart rate remained stable. Propranolol, 10 mg/kg iv, before CSD and 10 mg.kg-1.day-1 iv for 2 days prevented all ECG abnormalities. Atropine, 1 mg/kg iv, before CSD and 2.5 mg.kg-1.day-1 iv for 2 days had no effect. Isoproterenol, 0.02-0.06 microgram iv, after CSD produced further lengthening of the MAP and Q-T interval. Blood gases and electrolytes remained unchanged, and cardiac histology was unremarkable. These results suggest that CSD produces alterations in cardiac sympathetic activity in the rat leading to MAP and Q-T prolongations and to changes in the T wave form. Similar mechanisms may be operative after carotid endarterectomy in man.