Thirty days after resection of 10 to 80% of the midportion of the small intestine, excluding the duodenum, several cell kinetic parameters were investigated in the residual intestine. The degree of intestinal response increased in a stepwise fashion as the amount of tissue removed was increased. The response involved marked increases in: DNA synthesis per crypt expressed as disintegrations per minute of tritium (3H) reflecting (3H)thymidine incorporation, cells per crypt column, 3H-labelled cells per crypt column, cells per villus column, and thickness of all intestinal wall components. These changes occureed throughout the small intestine even at lesser resections. ""Crypt profiles'' reflected changes in cell counts, but when the labeling frequency of proliferative cells was expressed as a percentage of the total crypt height, there was no change. The total number of crypts in the duodenum remained unchanged and the total number of cyrpts in the residual jejunum plus ileum decreased proportionally to the amount of tissue removed. Intestinal compensation occurred by increasing the size of the structures present in the residual intestine, not by increasing the number of structural units.