Electron microscopic examination of samples from various regions of the rat small intestine was carried out. The number of mitochondria in the epithelial cells was estimated. The counts were made in sections of cells cut along their longitudinal central plane. The errors involved in extrapolating these counts to the whole cells were also estimated. The average mitochondrial number per cell section was 21 in the lower third of the crypts, it gradually increased in the mid and upper thirds and reached about double, 42, at the villus base. The known forms of dividing mitochondria were identified in the mid and upper third of the crypts. The counts remained around 42 along the epithelium of the villi. Crypt cells are continually produced in the lower crypt; these cells migrate to the villi while differentiating into nonproliferative absorptive cells. After inhibiting mitosis by methotrexate, this migration continued (Altmann, '74) and mitochondrial division persisted. In segments of the jejunum isolated surgically from the functional intestine for three weeks, mitosis and cell migration continued, but no evidence of mitochondrial duplication was found. Each mitochondrion probably undergoes a division as the crypt cells migrate from the mid-crypts to the villus. As a result, the villus epithelial cells contain double numbers of mitochondria. It appears that the mitochondrial division is not directly related to mitosis and is elicited by a stimulus present only in the functional intestine.