1. The intestinal absorption of a mixture of [ 3 H]cellobiotol and [ 14 C]mannitol was determined by measuring the 3 H/ 14 C ratio in urine after oral administration of the labelled sugars to rats. This index of intestinal permeability was used to identify cellular dysfunction in experimental enteropathy in rats. 2. Rats with mucosal damage induced with ethanol showed an increased uptake of [ 3 H]cellobiotol and a decreased uptake of [ 14 C]mannitol compared with normal controls. The increased 3 H/ 14 C ratio in urine reflected the abnormal cell function known to be caused by ethanol. 3. Methotrexate treatment reduced the absorption of the two sugars whereas cetrimide treatment enhanced their absorption. However, in both methotrexate-induced and cetrimide-induced enteropathy the 3 H/ 14 C ratio in urine was unaffected by the mucosal damage. Here the permeability change was not related to absorptive dysfunction of the mucosal cells. 4. It is concluded that this labelled sugar absorption test enables the rapid and accurate identification of malfunction of intestinal mucosal cells in the rat. Moreover, the test distinguishes between changes in permeability caused by abnormal cell function and changes caused by gross disturbance of the structure of the mucosal surface.