Abstract: The changes of middle molecular substances (MMS) content in plasma of rabbits following abdominal surgery have been assayed by an improved ultra-violet ray absorption method. The results have showed that MMS level declined continually and significantly in the abdominal surgery group; but no significant changes have been found in the controled group. The results suggest that abdominal surgery could make MMS level in plasma decline significantly. The authors consider that the mechanism of MMS level decline may be correlated with stress response.