In 14 patients with symptomatic high-degree AV block, studies were carried out of the effect on physical working capacity when an atrial triggered mode of cardiac pacing (VAT) was substituted for ventricular inhibited pacing (VVI). The tests were designed to elucidate the quantitative relationship between the effect of rate increase and AV synchronization by comparing VVI pacing matched to the same rate as VAT. The work load was increased by 10 W/min. The physical working capacity increased by 25% with VAT as compared with VVI (p less than 0.001). The improvement was entirely attributed to heart rate increase, and not to AV synchronization.