The central haemodynamics at rest and during exercise have been studied in 25 patients with complete AV block who were treated with fixed rate (FRP) and atrial triggered pacemakers (ATP). The aim of the investigation has been to study the effect of a synchronized atrial contraction for the filling of the ventricles and for the cardiac output (Q̇). Pressures and Q̇ have been determined during heart catheterization. The P wave for triggering the atrial synchronized pacemaker has been obtained with an electrode in close contact with the atria, introduced by means of mediastinoscopy. The study consists of two series. In the first series (12 patients) the central haemodynamics were recorded with each patient connected first the FRP (about 70 impulses/min) and later to the ATP. Most patients were studied both at rest and during exercise, the work loads being identical with both types of pacemakers. Q̇ at rest is 10% higher with ATP (p < 0.02) than with FRP and during exercise 20 % higher with ATP (p < 0.01). Stroke volume (SV) at rest is equal with both types of pacemakers, but significantly larger with FRP during work (p < 0.001). The left ventricular (LV) filling pressure is significantly lower (p < 0.01) with ATP at rest, but not during exercise (p < 0.8). In the second series 13 patients were studied at rest and during exercise. The recordings of pressures and Q̇ were first performed with the patient on ATP. After 30 min rest an identical study was performed with the patient connected to FRP, the rate of which was matched (FRPm) to that previously recorded with ATP. Q̇ at rest is 18% higher (p < 0.01) with ATP than with FRPm and during work 8 % higher (p < 0.05) with ATP. SV at rest is significantly larger (p < 0.01) with ATP than with FRPm, whereas during exercise no significant difference is observed between the two types of pacemakers. LV filling pressure at rest is significantly lower on the 5 % level with ATP; during exercise no significant difference is observed. The investigation shows that in many patients, especially younger ones, treatment with ATP makes it possible to obtain a larger Q̇ during exercise, and thus to increase the oxygen transporting capacity of the circulation. This benefical effect may be more pronounced in patients with low compliance of the ventricular myocardium.