In brief: The role of tennis in maintaining cardiovascular fitness has never been clearly evaluated. Therefore, we studied 28 intermediate male players between the ages of 45 and 72 who played tennis six to ten hours per week. The subjects were monitored before, during, and after singles tennis to determine the effect on heart rate and cardiac rhythm. Later 13 of the participants were tested on a treadmill to examine their oxygen consumption and physical condition. During an hour of tennis play, heart rate was maintained at a steady-state level between 60% and 100% of age-predicted maximum. These subjects also demonstrated fitness levels similar to those of persons who have run 2 1/2 hours per week for ten years.