An attempt was made to circumvent problems inherent in fixed-rate artificial cardiac pacemakers by utilizing an implantable demand pacemaker in 17 patients. Complete heart-block was present in 14, and 3 had other bradyarrhythmias. The cause was idiopathic in eight (mean age, 78.6 years), and was ischemic heart disease in nine (mean age, 67.7 years). All had Adams-Stokes attacks; ten had heart failure. Following implantation, 11 of 14 patients with complete heart-block exhibited transient return of idiocardiac rhythm, but competition did not occur. Two patients died; one of previous septicemia, and one death was unexplained. Six nonlethal complications included catheter displacement, threshold elevation, partial sensing failure, and postoperative hematoma. The frequent return of intrinsic cardiac rhythms and the occasional need for pacing in patients without complete heart-block emphasize the advantages of the demand pacemaker system.