Some pathological conditions may affect osmolarity which can impact cell, tissue and organ volume. The hypothesis of this study is that changes in osmolarity affect the zero-stress state and mechanical properties of the aorta. To test this hypothesis, a segment of mouse abdominal aorta was cannulated in vivo and mechanically distended by perfusion of physiological salt (NaCl) solutions with graded osmolarities from 145 to 562mOsm. The mechanical (circumferential stress, strain, elastic modulus) and morphological (wall thickness, WT and wall area, WA) parameters at loaded state were determined. To determine the changes of zero-stress state in response to osmolarity, the opening angle was observed by immersing the sectors of thoracic aorta in mouse, rat and pig into NaCl solution with different osmolarities. Moreover, wall volume and tissue water content of the rings at different osmolarities were recorded. Our results show that acute aortic swelling due to low osmolarity leads to increase in wall thickness and area, change in stress-strain relationship and increase in elastic modulus (stiffness) in the mouse aorta. The opening angle, wall volume and water content decreased significantly with increase in osmolarity. These finding suggests that acute aortic swelling and shrinking result in immediate mechanical changes in aorta. The changes of zero-stress state through osmotic pressure may serve to regulate mechanical homeostasis.