In 20 experiments the distensibility characteristics of venous microvessels of 22–148 μm internal diameter in response to arterial and venous pressure changes were examined microphotographically in the isolated and perfused mesentery of the dog. Over a range of arterial pressure between 0 and 170 mm Hg venular diameter changed by 31,8±8,8% and venular length by 6.3±4.4%. Venular length changes were significantly correlated to corresponding changes in the accompanying arterioles, whereas no correlation could be found between changes of length and diameter and the control diameters. With venous pressure elevation from 0 to 30 mm Hg an increase of the volume of venous microvessels of about 360% was measured; beyond a venous pressure of 30 mm Hg a limitation of distensibility was observed in these vessels. The moduli of volume elasticity calculated from these data were lower than the moduli reported for the total venous vasculature of the intestinal bed in the range of physiological pressures. It is concluded that the venous microvessels represent the most distensible elements of the venous vascular system.