We assessed intestinal structure, mucosal epithelial kinetics, and disaccharidase activities after fasting. Rats fasted for up to 120 hours were compared with control rats fed ad libitum. All rats had free access to water and all were prevented from ingesting their own stools. Body weight, small intestinal weight and mucosal protein, and maltase and sucrase activity of the total small intestine decreased in fasted rats. Lactase activity did not decrease. Specific activity of lactase actually increased in the jejunum. Assessed after a 96-hour fast, jejunal villi were shortened with less epithelial cells along their length and the rate of migration of those cells along the villi was diminished in the fasted rats compared with control rats. We attribute the decreased total intestinal sucrase and maltase activities to a loss of total epithelial cell mass in the small bowel. An abnormality in the cells of the progenitor zone of the crypts is suggested by the decreased migration rate of mucosal epithelial cells in fasting rats. These factors do not explain our observations completely since lactase activity did not diminish. We postulate that the activity of the "acid" β-galactosidase located in the cytoplasm or lysosomes of the epithelial cells was stimulated by fasting. Our observations are relevant to clinical pediatrics. Undernutrition and fasting my be associated with many childhood diseases and with treatment of disease. In assessing clinical data and advising treatment, the pediatrician should be aware of the potentially harmful effects of starvation on intestinal structure and function.