Hyperkalemia may cause cardiac pacemaker (PMK) malfunctioning due to a reduction of the electronegativity of the resting myocardial potential. Both sensing and capture mechanisms could be temporarily affected, with possible life-threatening effects. Calcium chloride may counteract almost immediately such a dangerous effect of hyperkalemia, restoring the normal pacemaker function. We describe the case of a 75-year-old woman, observed in the Emergency Department for extreme weakness, very large QRS complexes, and a pacemaker type II exit block, associated with sensing failure and "spike-on-T" phenomenon. She was managed with immediate i.v. calcium chloride, followed by insulin/glucose and sodium bicarbonate infusions; the ECG recordings show an almost immediate correction of the PMK malfunctioning and serial improvement of the intraventricular conduction. This case supports the feasibility and effectiveness of i.v. calcium administration, as expected on the basis of electrophysiological ionized calcium effect on the threshold potential.