A cardiac pacemaker including a pulse generator and an electrical source such as a battery completely enveloped in a small size housing. An electrode operatively connected to the pulse generator has a helically-shaped portion extending outwardly from the housing in fixed relation thereto. The housing is of sufficiently small size to permit the pacemaker to be implanted in conjunction with a limited thoracotomy by turning the housing to rotatively insert the electrode into the heart tissue of the patient, i.e. into the myocardium. The housing can be of electrically conducting material, insulated from the electrode, and a portion of the housing can serve as an indifferent electrode.