The relation between the occurrence of repolarisation abnormalities after right ventricular pacing and spontaneous arrhythmias was investigated in 16 patients in whom the sick sinus syndrome was suspected. All patients had normal QRS complexes and T waves in the electrocardiogram before pacing and required atrial stimulation and His bundle recording for diagnostic purposes. Patients were randomised into a study group or a control group. In the eight patients in the study group right ventricular pacing was performed for 12 hours, and was followed by inversion of the T wave in surface leads II, III, aVF, and V2-V5 and lengthening of the QTc interval. The frequency and complexity of ventricular arrhythmias increased after pacing in six patients who had ventricular extrasystoles in the baseline Holter recording. As the configuration of the T wave became normal the frequency of ventricular extrasystoles returned to baseline values. In the control group of eight patients ventricular pacing was not performed after the electrophysiological study and no changes were seen in T wave configuration and in the frequency of spontaneous arrhythmias. These results suggest that the post-pacing repolarisation abnormalities reflect abnormal electrical properties of the ventricle and that in some cases they lead to increased electrical instability.