The effects of chronic increase in aortic blood flow on arterial wall remodeling were investigated in vivo with the use of an aortocaval fistula (ACF) model in rats. Phasic hemodynamics and aortic wall structure upstream and downstream in 30 male Wistar rats with ACF and 30 sham-operated rats were compared immediately and 2 mo after the ACF was opened in anesthetized rats. Opening the ACF upstream acutely decreased aortic pressure (-30%, P < 0.001) and increased aortic blood velocity (x12, P < 0.001), blood flow (x9, P < 0.001), wall shear stress (x10, P < 0.001) and guanosine 3',5'-cyclic monophosphate (cGMP) wall content (+50%, P < 0.01). After 2 mo, aortic pressure decreased (-22%, P < 0.001) and aortic blood velocity, diameter, and blood flow increased (+114%, P < 0.001; +60%, P < 0.001; and +250%, P < 0.001; respectively) compared with the control group. Aortic wall shear stress and cGMP wall content dropped over time and tended to recover control values; aortic wall tensile stress was higher than in the control group (P < 0.05). Medial cross-sectional area and elastin and collagen contents increased (+38%, P < 0.01; +50%, P < 0.01; and +30%, P < 0.05, respectively) and were associated with smooth muscle cell hypertrophy) (+23%, P < 0.05), despite a decrease in arterial wall thickness (-13%, P < 0.01). Opening the ACF downstream acutely decreased aortic pressure (-30%, P < 0.001) without any change in aortic blood velocity, diameter, blood flow, shear stress, and cGMP wall content. After 2 mo, pressure, blood velocity, shear stress, and cGMP wall content decreased (-22%, P < 0.001; -31%, P < 0.01; -46%, P < 0.02; and -50%, P < 0.05; respectively) and diameter and blood flow were unchanged; smooth muscle cell hypertrophy and hypoplasia were the only observed changes in the aortic wall structure. These results suggest that both shear and tensile stresses are involved in the aortic wall remodeling. Increase in shear stress likely induces expansive remodeling in relation to flow-dependent vasodilation, whereas increase in tensile stress is responsible for medial hypertrophy and fibrosis.