Introducing a universal tensile testing instrument into medical investigation, cyclic stretch and release and stress -relaxation tests were performed on the excised walls of the toad aorta and vena abdominalis. Hysteresis loop of the aorta was far less remarkable than that of the vena abdominalis as expected. Changes in the initial length setting affected the ascending limb of the hysteresis and that of the final length, the descending limb, in such a way that an increase of the setting length caused an increment of the steepness of respective limbs. Changes in the loop under variable conditions such as the rate, number and interval of the cyclic stretch with a "definite amplitude" were mainly produced by alteration of the ascending limb, while the descending limb appeared rather stable. Electrical stimulation of the vessels enhanced the hysteresis loop and its steepness mainly elevating the ascending limb. Formic acid digestion of the vessels produced a decrease of elastic modulus and a dramatic disappearance of the hysteresis, whereas trypsin digestion caused an increase of the former accompanied by a considerable size of hysteresis. Thus, the ascending limb seems to relate mainly to the muscle function, the descending upper half limb to the collagen and the lower half to the elastin elements. Stress-relaxation curves for different grades of stretch were analysed into 3 component periods from the distribution functions, i. e. the initial, middle and last rising phases. Comparison of the normal distribution functions to those of formic acid and trypsin treated ones clarified that the collagen element played a main role at the initial phase, the muscle at the middle and the elastin at the last phase.