Many physiological investigations of cardiac function require accurate control of heart rate. To examine heart rate effects at values lower than the intrinsic heart rate requires the ability to prevent an accelerated junctional rhythm from becoming dominant even after the sinus node is eliminated. A new method for the simple and reliable production of complete atrioventricular heart block in open-chest dogs is presented. It consists of two bonded parallel 22-gauge needles, one containing a bipolar recording electrode for localization at its tip and the other used for immediately adjacent formalin injection. The catheter is easily constructed, sterilizable, reusable, and durable, and can also provide a stable means of recording the His bundle electrical activity. Initial experiments performed in the first five dogs using this technique produced complete heart block with a stable ventricular escape rhythm in all five cases.