Increased sitting time is associated with increased arterial stiffness, poor cardiometabolic health outcomes, increased risk of developing cardiovascular disease, and all-cause mortality. Further, excess sitting time likely reduces aerobic capacity and increases adiposity, which have been shown to be related to low testosterone (T) in young men. Low T has been linked to increased arterial stiffness, however, it remains unclear how T may contribute to the associations between physical inactivity, adiposity, aerobic capacity, and vascular health. PURPOSE: To determine the influence of T on the relationships between VO2max, body composition, physical activity, endothelial function, and arterial stiffness. METHODS: 87 healthy male adults aged 18-75 years (mean±SD; 53±14 years, BMI=27.6±4.4 kg/m2) underwent VO2max testing, physical activity monitoring (accelerometry), body composition (DXA), and vascular (endothelial function via brachial artery flow-mediated dilation, FMD; arterial stiffness via pulse wave velocity, PWV) testing. Serum T was measured under fasted conditions in the morning. RESULTS: Bivariate correlation analysis indicated that VO2max (p=0.003, R=0.33), body fat (p<0.001, R=-0.42), sitting time (p=0.020, R=-0.28), PWV (p=0.049, R=-0.22), and FMD (p=0.046, R=0.24) were related to T concentrations. Body fat (p<0.001, R=-0.39), VO2max (p<0.001, R=0.41), and sitting time (p=0.029, R=0.24) were correlated with PWV. Body fat (p=0.001, R=0.44) and VO2max (p=0.001, R=-0.41), but not sitting time (p=0.185, R=-0.17), were related to FMD. After controlling for T using partial correlation analysis, sitting time was no longer significantly related to PWV (p=0.297, R=0.141), however correlation coefficients between PWV and VO2max or body fat were unchanged. CONCLUSIONS: These data indicate that the association between physical inactivity may be related to low T concentrations.