The presence of testosterone (along with functional androgen receptors), especially during growth and development or periods of enhanced training leads to fundamentally different cardiovascular adaptations to endurance training in males compared to females.  Males have larger hearts, both in absolute terms, and when scaled to lean body mass.  In longitudinal studies, even when matched heart beat x heart beat for a full year of progressive training, young men evolve larger hearts, and greater increases in aerobic power.  In this talk, I will review the cardinal features of the cardiovascular adaptation to endurance training in competitive athletes, and then review the data in longitudinal studies.  The implications of these differences in cardiovascular adaptation for endurance performance will be discussed as part of the biomedical basis for elite performance differences between males and females.