The acute and long-term hemodynamic benefit from atrial synchronization in ventricular (VAT) pacing has been investigated at rest and during exercise in 10 patients undergoing pacemaker implantation for complete A-V block. The results were compared to conventional (VOO) ventricular stimulation at rates of 70 BPM and 96 BPM. Cardiac index (CI) in VAT-pacing increased at rest by 8% and during exercise by 15% more than with VOO pacing (p less than 0.01). No significant change between the two different rates of asynchronous pacing was observed. CI at rest and during exercise was unchanged after 10 weeks of VAT-pacing. CI is regulated by change of stroke volume in VOO-pacing and by heart rate only with VAT-pacing. In contrast to earlier results with asynchronous ventricular pacing, the hemodynamic benefit of pacing in an atrial synchronized mode is long-lasting. Physiologic mechanisms regulate hemodynamics during exercise and in heart failure.