The transitory laxative threshold of a partially digestible disaccharide, trehalose, and an undigestible disaccharide, lactulose, was estimated by the dose-response relation between the test substance and the prevalence of diarrhea in 20 healthy female subjects. The subjects ingested several indicated amounts of trehalose or lactulose once daily 2 to 3 h after a meal. The intake of the test substance was stopped at the dose level that caused diarrhea or when the dose reached the maximal level. A record of physical conditions, gastrointestinal symptoms, and fecal conditions was made by all subjects before and after each ingestion of the test substance. Half the subjects experienced no diarrhea even with the ingestion of the maximal dose level (60 g) of trehalose in this study, and the ingestion of up to 40 g of lactulose caused diarrhea in 75% of all subjects. Abdominal symptoms such as flatus, distension, and borborygmus appeared at high prevalence with lactulose and trehalose ingestion, and the effect of lactulose was significantly stronger than that of trehalose at the same dose level (p<0.05). The quantity of trehalose and lactulose that induced diarrhea differed greatly from person to person. The transitory laxative threshold was estimated as 0.65g/kg body weight for trehalose and 0.26g/kg body weight for lactulose by using the regression equation between the dose levels of the test substances and the cumulative incidence of diarrhea. These results suggest that it would be quite acceptable to administer trehalose up to 33 g and lactulose up to 13 g in a person weighing 50 kg.