We performed atrial EP studies (atrial substrate evaluation) on 10 patients. These patients had evidence of paroxysmal, sustained, recurrent atrial arrhythmias (7 men and 3 women with a mean age of 64 +/- 15 years). All patients combined a brady-tachy syndrome; 7 patients had a sick sinus syndrome (SSS) and 3 patients a typical vagally induced atrial arrhythmia. No anti-arrhythmic drug was allowed in 3 patients with SSS, 1 drug failed in 4 patients and the combination of 2 drugs failed in 3 patients during the first to eighth years prior to pacemaker implantation. Atrial substrate evaluation was feasible in all these patients off anti-arrhythmic therapy and showed important abnormalities of atrial loco-regional conduction parameters and long refractory periods (RP). The remarkable point was, in 7 patients, a paradoxical improvement in intra-atrial conduction delay at rapid pacing rate. The DDD pacing mode was chosen in all patients. No technical problem occurred during implantation. Atrial pacing rate was programmed to be slightly higher than the mean diurnal heart rate calculated on Holter monitoring. After implantation, the mean follow-up period was 18 +/- 25 months with an average of one Holter every 4 months during the first 2 years. The 7 patients who improved intra-atrial conduction at rapid pacing rate were controlled without drugs, 2 patients were controlled with 1 drug, and 1 patient with 2 drugs. Atrial pacing in the DDD mode in a selected group of patients prevents paroxysmal and drug-resistant atrial arrhythmias. Atrial substrate evaluation is a sensitive tool for assuring the long-term benefit of atrial pacing.(ABSTRACT TRUNCATED AT 250 WORDS)