Ten elderly patients each had a ventricular rate responsive pulse generator (Activitrax) placed in them to help correct severe conduction abnormalities; none had suspected ischemic heart disease. The pulse generator was programmed to a maximal pacing rate of 125 ppm, a medium activity threshold, and a rate response of 6. Six weeks after implantation of the pulse generator, the patients were evaluated before exercising and again when the pacing rate reached 125 ppm. The evaluation protocol included an M-mode echocardiogram from which the following measurements were taken: the left ventricular end-diastolic volume (EDV), the end-systolic volume (ESV), the ejection fraction (EF), and the peak systolic pressure/end-systolic volume (PSP/ESV). The numerical values were recorded, calculated, and compared statically with the following results: the EDV increased from 91 +/- 10 to 125 +/- 20 cc (p less than .05); the ESV decreased from 64 +/- 10 to 24 +/- 6 cc (p less than .005); the EF increased from 41 +/- 5 to 61 +/- 10% (p less than .05); and the PSP/ESV ratio increased from 1.70 +/- 1 to 4.10 +/- 2 mm Hg/cc (p = 10). Also, during the maximal pacing rate, the septum of all patients showed paradoxical septal motion. All patients in our study have been asymptomatic and have shown an increase in their exercise capacity. We conclude that during exercise the left ventricular function ins influenced more by heart rate than by AV synchrony, as indicated by an elevated EDV in most patients.