Seventeen patients were followed up for a period ranging from 12 to 40 months after implantation of an Elema Vario pacemaker with an endocardial electrode. Two patients had a greater stimulation threshold beyond 6 months after implantation than the maximum threshold recorded before 6 months. One of these patients required replacement of the pacing system because of exit block. The 16 patients who retained their pacing systems showed, as a group, a statistically significant fall in stimulation threshold at 12 months after implantation despite continued pacing. These findings are important in relation to the occurrence and prediction of late exit block when long-life impulse generators are used.