Gated radionuclide ventriculography was performed in 53 patients, with 29 DDD and 24 VVIR pacemakers. Ejection fraction and regional contractility was studied in three conditions: a) At rest. b) With tachycardia after exercise. And c) With induced tachycardia at rest. At a rate similar to the one reached with the exercise. Stimulation in different parts of the right ventricule did not show any differences in the ejection fraction. Induced ventricular stimulation in comparison with natural ventricular contractions did not alter or change ejection fraction if the reached rate was the same an not too high in both cases. Induced stimulation at rest with a rate of 110-120/min, decreased the ejection fraction in 10% (p less than 0.001). Induced stimulation by exercise (VVIR. DDD) increased the ejection fraction in 10% (p less than 0.003) and if there were zones of dyskinesia they improved or disappeared. There were no statistical differences between VVIR and DDD pacemakers. It seems that a limited tachycardia is convenient for patients with rate response pacemakers (VVIR).