['Physical fitness and level of regular exercise are closely related to cardiovascular health. A regimen of regular intensity-controlled treadmill exercise was implemented and withdrawn to identify cellular mechanisms associated with exercise capacity and maximal oxygen uptake (VO2max).', 'Time-dependent associations between cardiomyocyte dimensions, contractile capacity, and VO2max were assessed in adult rats after high-level intensity-controlled treadmill running for 2, 4, 8, and 13 weeks and detraining for 2 and 4 weeks. With training, cardiomyocyte length, relaxation, shortening, Ca2+ decay, and estimated cell volume correlated with increased VO2max (r=0.92, -0.92, 0.88, -0.84, 0.73; P<0.01). Multiple regression analysis identified cell length, relaxation, and Ca2+ decay as the main explanatory variables for VO2max (R2=0.87, P<0.02). When training stopped, exercise-gained VO2max decreased 50% within 2 weeks and stabilized at 5% above sedentary controls after 4 weeks. Cardiomyocyte size regressed in parallel with VO2max and remained (9%) above sedentary after 4 weeks, whereas cardiomyocyte shortening, contraction/relaxation- and Ca2+-transient time courses, and endothelium-dependent vasorelaxation regressed completely within 2 to 4 weeks of detraining. Cardiomyocyte length, estimated cell volume, width, shortening, and Ca2+ decay and endothelium-dependent arterial relaxation all correlated with VO2max (r=0.85, 0.84, 0.75, 0.63, -0.54, -0.37; P<0.01). Multiple regression identified cardiomyocyte length and vasorelaxation as the main determinants for regressed VO2max during detraining (R2=0.76, P=0.02).'] Is aerobic fitness associated with cardiomyocyte contractile capacity and endothelial function in exercise training and detraining?