Although physical activity (PA) improves arterial distensibility, it is unclear which type of activity is most beneficial. We aimed to examine the association of different types of PA with carotid distensibility (CD) and the mechanisms involved. Data included 4503 Australians and Finns aged 26-45 years. Physical activity was measured by pedometers and was self-reported. CD was measured using ultrasound. Other measurements included resting heart rate (RHR), cardiorespiratory fitness (CRF), blood pressure, biomarkers and anthropometry. Steps/day were correlated with RHR (Australian men r=-0.10, women r=-0.14; Finnish men r=-0.15, women r=-0.11; Po0.01), CRF and biochemical markers, but not with CD. Self-reported vigorous leisure-time activity was more strongly correlated with RHR (Australian men r=-0.23, women r=-0.19; Finnish men r=-0.20, women r=-0.13; P<0.001) and CRF, and was correlated with CD (Australian men r=0.07; Finnish men r=0.07, women r=0.08; P<0.05). This relationship of vigorous leisure-time activity with CD was mediated by RHR independently of potential confounders. In summary, vigorous leisure-time PA but not total or less intensive PA was associated with arterial distensibility in young to mid-aged adults. Promotion of vigorous PA is therefore recommended among this population. RHR was a key intermediary factor explaining the relationship between vigorous PA and arterial distensibility.