Cardiac Output and Pacing Rate.Introduction: The purpose of this report is to determine the optimal pacing rate for an exercising patient.Methods and Results: From a review of the literature and from our own animal studies, the relationship between cardiac output and pacing rate and that between stroke volume and pacing rate are examined in resting and exercising animal and human subjects. With an adequately wide range of pacing rates, there exists a three‐phase relationship between cardiac output and pacing rate. Starting with a low pacing pacing rate, an increase in pacing results in an increase in cardaac output (phase 1), then little change in cardiac output (phase 2), after which there is a decrease in cardiac output (phase 3). However, the relationship between stroke volume and pacing rate does not typically exhibit characteristics that allow identification of the three phases. In resting subjects with impaired ventricles and in exercising subjects, phase 2 is narrow or absent, the cardiac output increasing, then decreasing with an increase in pacing rate. From the experimental data reviewed herein, a technique is proposed for identifying the starting point in selecting the best pacing rate for resting and exercising subjects. The technique relies on identification of the transitions in the three‐phase relationship between cardiac output and pacing rate.Conclusion: The best resting pacing rate is at the lower end of phase 2. A tentative first‐choice exercise rate is at the middle of the resting phase 2 region. However, a change in cardiac output with a change in pacing rate will allow refinement of this first‐choice exercise paci