An apparatus for stimulating tissue of a medical patient includes a power transmitter which periodically transmits a pulse of a radio frequency signal to a intravascular stimulator that is implanted in a vein or artery. The intravascular stimulator employs energy from the radio frequency signal to charge a storage device which serves as an electrical power supply. The intravascular stimulator also detects an electrical signal produced within the patient and responds thereto by applying a pulse of voltage from the storage device to a pair of electrodes implanted in the vascular system of the animal.