A satisfactory technique for the continual infusion of solutions into the intestinal tract of the small animal was developed. Using this technique, dilute solutions of lactic acid were infused into the rat and their effect on fat absorption and mucosal structure assessed. Thirty-five rats were studied in five groups, infused with either ion-free water or lactic acid (2.0 g/100 ml, 2.5g/100 ml, 3.0 g/100 ml, 3.5 g/100 ml) for 10 days. Fecal fat excretion measured as a percentage of ingested fat was increased significantly in rats infused with acid in a concentration of 3.0 g/100 ml (7.2 ± 2.1) and 2.5 g/100 ml (6.5 ± 2.1) compared to the water-infused group (2.9 ± 1.9) or those infused with lactic acid 2.0g/100 ml (2.2 ± 0.9). Six rats infused with lactic acid 3.5 g/100 ml died within 72 hours of a perforated intestine and the two remaining ones were killed prematurely for histological study.Histological abnormalities were confined to the small intestine. Mucosal structure was normal in both the water-infused group and those receiving lactic acid 2.0g/100 ml. Shortening of villi occurred at the site of infusion in both the groups infused with lactic acid 2.5 g/100 ml and 3.0 g/100 ml. Fusiform processes of epithelial cells were observed extending on narrow pedicles from the tips of some of the shortened villi. Gross derangement of villous structure was observed in two rats killed after 48 hours on the most concentrated acid (3.5 g/100 ml).It is probable that the abnormalities in fat absorption and mucosal structure were a direct effect of the infusion with lactic acid. The mechanism behind the development of steatorrhea can only be the subject of speculation. The histological observations suggest that the lesions produced in the mucosa were associated with disordered turnover of the surface epithelial cells.