The experiments were performed on adult rabbits divided into four groups; simple laparotomy, small intestinal anastmosis, resections of upper and lower small intestines (1/3 to 1/2 of the whole length, respectively), estimating inorganic substances and protein in the sera for a week after operation. The function of the parathyroid glands was pursuited histologically after operation. 1) Calcium, potassium and protein in the sera were decreased in amount after operation. The decrcasing rate was highest in the group resected upper small intestine, the lower, the anastomosed, and the simply laparotomized in order. The rate was maximum on the second day after operation, and thereafter increased gradually until nearly normal level on the seventh day. 2) The amount of inorganic phosphorus in the serum was in inverse proportion with calcium, and was increased after operation, maximum on the second and returned to normal on the seventh postoperative day. The increasing rate was highest in the group resected upper small intestine, the lower, theanastomosed, and the simply laparotomized in order. 3) In the rabbit resected small intestine showed, histologically, a picture of hyperfunction the parathyroid gland, which was most remarkable in the cases passed three weeks and in two months after operation.