Although the majority of heart transplant recipients have a satisfactory heart rate, a substantial number require a permanent pacemaker. In 7 of 46 heart transplant patients at our institution symptomatic bradycardia developed, necessitating implantation of a transvenous pacemaker. The average time from heart transplantation to pacer insertion was 25 days. The average donor age, ischemic period, and crossclamp time was 28 years, 182 minutes, and 113 minutes, respectively. A long aortic crossclamp time (greater than 83 minutes) increased the risk for conduction abnormalities in the sinoatrial node. No patient had rejection before the pacer implantation. Five of the seven patients continue to be paced a significant amount of a 24-hour period. Only one patient has had considerable improvement in 3 years, requiring pacing only 3% of a monitored 24-hour period. This patient had the longest ischemic time and the most rejection episodes after implantation of the pacemaker. One patient was paced 100% until a second heart transplantation was done, without a subsequent need for pacing. The other five patients' hearts continue to be paced between 80% and 100% of a 24-hour monitored period. The donor intrinsic heart rates of these five patients produce symptomatic bradycardia. The success of AAI pacing in all patients indicates normal conduction below the sinoatrial node. The injury or dysfunction resulting in bradycardia was isolated to the sinoatrial node. Long-term follow-up in three patients (greater than 3 years) shows the need for pacing to be intermittent but long term. Most patients never fully recover from symptomatic bradycardia.(ABSTRACT TRUNCATED AT 250 WORDS)