Sixteen patients with a non-invasively programmable pacemaker (Medtronic 2409) were examined with the unit in ventricular inhibited (VVI) and atrial synchronous ventricular inhibited (VDT/I) mode, respectively. Maximal exercise capacity was determined by means of bicycle ergometry. Work load was increased in steps of 10 watts each minute. Atrial and ventricular rates, brachial artery pressure, and respiratory rate were studied at rest and during exercise while systolic time intervals were determined at rest. There was an important increase in maximal exercise capacity when changing from VVI to VDT/I pacing. The extent of improvement was the same for patients above and below the age of 65 years. At comparable work loads VDT/I pacing resulted in significantly lower atrial rates than VVI pacing. Systolic time intervals did not differ between VVI and VDT/I pacing apart from an increase in left ventricular ejection time index with VDT/I pacing. Systolic time intervals and maximal exercise capacity with VVI pacing did not correlate with the increase in exercise capacity induced by VDT/I pacing. Physical performance can be significantly improved by VDT/I pacing in both young and old patients. Exercise capacity on VVI pacing cannot predict the possible benefit of change to VDT/I pacing.