Dual chamber DDD pacing is fully physiologic when chronotropic response of sinus node to exercise is normal and when retrograde ventriculo-atrial conduction is absent. Comparison of results from exercise test with increasing work load showed that atrial-triggered ventricular pacing provides a significant functional benefit (delta VO2 15%) P less than 0.01, if compared with fixed rate ventricular pacing. The benefit is closely related with the amount of sinus rate increase during exercise. In patients with sinus node syndrome the atrial triggered ventricular pacing rate did not show significant increase during exercise and exercise capability was similar to that observed with fixed ventricular pacing. Retrograde ventriculo-atrial conduction was observed in 56% of patients with sick sinus syndrome and 28% of patients with complete AV block and was the reason for endless loop tachycardias (ELTs). ELTs can be eliminated by lengthening atrial refractory period (ARP). In patients with ARP greater than or equal to 250 ms (47%), mild (8:7, 4:3) or important (2:1) AV block appeared during exercise test, with sudden drop of pacing rate and cardiac output at highest work load. Among other "physiologic" pacing modes, respiration traced ventricular stimulation showed high physiologic sensitivity (90%) and haemodynamic benefit comparable to that obtained during dual chamber pacing and without the related disadvantages.