A case of 80-year-old woman with marked hyperkalemia in the course of chronic treatment with angiotensin converting enzyme (ACE) inhibitors and spironolactone is presented. AAI pacemaker was implanted three months ago. The ECG revealed a nodal rhythm, ineffective AAI pacing and other typical features of hyperkalemia. After normalisation of potassium level the ECG changes disappeared. Potential mechanism of the nodal rhythm during ineffective atrial pacing is discussed.