Transmucosal movements of water and electrolytes were measured in male Wistar rats, using the intestinal perfusion technique after a single intramuscular injection of 40 mg/kg BW of 5-fluorouracil. These movements were studied, daily during 6 days after the injection, in relation to the histological study of mucosa. The results (1) may explain the diarrhoea observed in patients treated with this antimitotic agent; (2) suggest the existence of a secretion in crypts, followed by an incomplete reabsorption by villi; the secretion of crypts or the villus reabsorption are alternatively the driving factor depending on the reciprocal size of villi and crypts.