The distensibility of the major arteries has been investigated extensively, but the value expressed as Young Modulus varies widely by the different schools of the investigators, the major reason undoubtedly being the difficulties encountered in the measurement. In the present study, an attempt was made to elucidate the distensibility of the external carotid artery of the rabbit, which was placed in saline immediately after removing from the apparently healthy, normal rabbit without anesthesia. The circular section strip and longitudinal section strip were made from the whole artery, and Young Modulus of the whole artery, circular section and longitudinal section strips was calculated from the length-tension curve of each sample. Also, the similar samples of the artery seperately obtained were placed in ATP solution in the concentration of 0.15 mM and 0.30 mM, and Young Modulus was similary calculated. Experiments were performed at 15,45 and 75 min after the artery was removed from the rabbit, and the results thus obtained are summarized as follows. 1) Young Modulus of the whole external carotid artery of the rabbit in saline was 4.74 X 10⁷dyne/cm² at 15 min, but lower values were obtained at 45 and 75 min, Young Modulus being 4.62 X 10⁷dyne/cm² and 4.13 X 10⁷dyne/cm², respectively. When the arterial samples were placed in ATP solutions, Young Modulus did not change much throughout the experiment, and lower Young Moduli were obtained in 0.30 mM ATP solution than in 0. 15 mM ATP solution. 2) Young Modulus Of the Circular Section Strip in Saline was 4.11 X 10⁷dyne/Cm², 3.75 X 10⁷dyne/cm² and 3.90 X 10⁷dyne/cm², respectively, at 15, 45 and 75 min, the value at 15 min being the highest. However, when the strip was placed in ATP solutions, no appreciable change was observed throughout the experiment, and Young Moduli were lower in 0.30 mM ATP solution than in 0.15 mM ATP solution. 3) Young Modulus of the longitudinal section strip in saline was 2.12 X 10⁷dyne/cm², 2.48 X 10⁷dyne/cm² and 2.46 X 10⁷dyne/cm², respectively, at 15, 45 and 75 min, Young Modulus being slightly elevated in the latter part of the experiment. A similar tendency was observed when the strip was placed in ATP solutions.