A certain amount of evidence suggesting that the intestinal sucrase activity in rats plays an important role in the digestion and absorption of dietary sucrose has been obtained from many investigations (1-6). Little, however, indicates the physiological role of this enzyme activity in the nutritional condition at the level of the whole body. The present study was conducted to observe the effect of tris(hydroxymethyl)aminomethane (IRIS), an inhibitor of intestinal disaccharidase activity (7), added to a diet on changes in body weight, food consumption, fecal consistency and intestinal sucrase activity, and the effect of TRIS on digestive enzyme activities localized in the brush border membrane of the small intestine (8) in vitro. And from these experimental results the nutritional significance of intestinal sucrase activity in rats was discussed.Male rats of the Wistar strain were used, and the basal diet was a 20% casein high-sucrose diet (SCD) described in the previous study (9). Rats were fed on the basal diet ad libitum before initiation of the experiments. Thereafter they were fasted for 2 days, being refed on the experimental diet for 3 days. The experimental diets were SCD, the 20% casein high-starch diet (STD), and the 20% casein high-glucose diet (GD), the sucrose in SCD being replaced by a-starch and glucose respectively. The enzyme activities were determined in the manner described in the previous study (10). Statistical analysis was done by taking the least significant difference calculated according to the method of Snedecor and Cochran (11).In the preliminary experiments, the addition of TRIS at levels of 1 and more to SCD caused significant growth retardation, reduction in food consumption and diarrhea. As shown in Table 1, in experiment 1 the addition of TRIS at the 2% level to SCD produced significant reduction in body weight gain and food consumption, and diarrhea, whereas the addition of TRIS at the same level to STD did not produce any adverse effect, although changes in body weight and food consumption 485