Abstract: A wristwatch size cardiac monitoring apparatus which is worn on the wrist of one arm which in a first mode use a conductive material housing as one electrode which is held firmly in contact with the wrist of the arm on which it is worn. A finger of the other arm is placed on a receiving electrode of the casing which acts as a second electrode. Together, these two electrodes provide a lead I electrocardiac signal to the electronics within the housing. The monitoring apparatus can operate in a second mode when an external belt and set of sensors is plugged into a jack in the side of the housing. The set of sensors are positioned using the belt on opposite sides of the patient's chest just below the pectoral muscles and in contact with the skin. Additionally, there is described circuitry which is used to operate the rate monitor with either the internal or the external sensors.