Crypt population kinetics in both small bowel and colon were determined in the early postoperative period in the rat after either sham resection or resection of 50% of the midportion of the small bowel. A proliferative response was evident in the colon within 24 hours. This response of the labeling index was statistically significant in the left colon by comparison with the sham-resected animals, whereas in the right colon it was significant by comparison not only with the sham-resected but also with the intact animals. A proliferative response was documented in the remnants of the small bowel on the third postoperative day by comparison with those of the sham-resected animals. These findings indicate that adaptive hyperplasia begins very promptly in the intestinal remnants after partial resection.