Villus tissue osmolality and fluid and electrolyte transport were measured in intestinal segments exposed to cholera toxin. The osmolality of the luminal fluid was kept at about 100, 300, or 600 mOsm X kg-1 by use of appropriate concentrations of mannitol. A net fluid secretion was seen in all experiments, the magnitude being dependent on the osmolality in the lumen. A secretion of sodium, potassium, and chloride was also seen in all experiments but the secretion rate of electrolytes was independent of the osmolality in the intestinal lumen. The hydraulic conductivity of the villus epithelium, calculated from the lumen and tissue osmolality, was the same as that estimated in the normal intestines. A villus tissue osmolality gradient was apparent in all experiments regardless of the mannitol concentration in the lumen, the tip osmolality being hypertonic while the tissue osmolality at the base was isotonic. This was the case also when the luminal fluid was hypotonic, a finding opposite to what we found in an earlier study on the normal feline intestine. A likely explanation for this observation is that the crypts of Lieberkühn secrete fluid containing sodium chloride, which is absorbed by the villus epithelial cells. Hence, a luminal "circulation" of electrolytes between crypts and villi was suggested in the present experimental circumstances.