This article describes the characteristics of a new implantable pacemaker controlled by right atrial oxygen saturation and reports the first clinical experience in man. During the observation period over 5 months, there was no evidence of malfunction due to tissue growth. The system's reaction to exercise changes proved to be quick (5 s to 17 s); decay times varied depending on the magnitude of the power previously performed. Under various exercise tests, the rate increase was linear to the stress load. Measurements of cardiac output showed the system's hemodynamic feedback and its potential self-optimization of pacing rate.