The mechanical response to external loading of a particular soft extensible tissue, the cat's mesentery, has been determined.First the hysteretic behavior of thein situ mesenteric membrane was measured and compared with the response of excised membrane. The response of the lving tissuein situ differed markedly from that of the excised tissue both with respect to the magnitude of response and with respect to ability to recover following a cycle of loading. It was observed that the tissue continuously degraded, i.e., suffered a decrease in modulus, if the overall sequence of loading cycles was continuous. However, with a recovery period of about 5 min between loading cycles, thein situ tissue would either completely or largely recover its initial cycle response. The excised tissue did not show significant (i.e., 0–15%) recovery in a coressponding time interval. The tension in the tissuein situ was not unique but could vary significantly according to the physical condition of the membrane per se as well as the state of the gut and the large blood vessels. The most nearly unique length of the tissue which could be measured by these experimental methods was a relaxed length determined by excising a rectangular piece of tissue of known dimensions and then immediately measuring the freely floating (in a physiologic solution) dimensions to which the tissue relaxed. There was no marked material anisotropy in the plane of the membrane, i.e., two principal dimensions in the plane of the excised mesentery did not vary by more than 5% even with wide history variations just prior to excision. This relaxed length of the excised tissue has been used in normalizing the stretch in the hysteresis plots for the tissue. Photomicrographs of the living tissue were taken during the course of the mechanical loading cycles. These concomitant observations on structural changes (visible in the light microscope) were correlated with the force-stretch plots to determine the extent to which they could be used to explain the hysteresis.