The main purpose of the present work was to study the influence of muscular activity on the dynamic behavior of the carotid artery. To this purpose we determined the input impedance of the vessel as a function of frequency. On the other hand, we calculated such impedances theoretically for an elastic tube model closed at the peripheral end, using different parameters such as speed of propagation, damping constant and length. Even a mere qualitative comparison of the experimentally obtained with the calculated impedance-curves yields the following interesting results: if we cancel the muscular activity by injection of papaverine, the carotid clamped at the peripheral end shows a picture which closely ressembles the theoretical one. Also a natural clamped carotid, not influenced by any drug, still shows some general features of the theoretical picture. On the other hand, a striking difference can be observed under the influence of the pressure reducing polypeptide firstly obtained by Laszt . In this case, phase shifts between pressure and flow are largely enhanced beyond the theoretically calculated limits from –90° to +90°. These large phase-shifts can only be accounted for by a negative damping constant β. This, in turn, means that we must introduce in the theory some kind of an amplifying element due to muscular activity. A further interesting result can be obtained if we record pressure and flow shortly after the removal of the clamp. In this case, the impedance-curves show the behavior of a tube with an open end not encountered normally in the arterial circulation. This might be attributed to reactive hyperemia of the peripheral vessels.