The influence of hydrolysis on the assimilation rate of important nutritional carbohydrates was studied in healthy subjects and patients with intestinal diseases, mainly by means of 13CO2 breath test techniques. All substrates were "naturally enriched" with carbon-13. The studies showed that hydrolysis is the rate limiting step for the assimilation of lactose, starch and even maltose, but not for the assimilation of sucrose. The degree of gelatinisation and the degree of side-branching of starch molecules were two important parameters, influencing starch hydrolysis in normal subjects. Addition of wheat bran had no influence on the digestion rate of starch. A comparative study between normal subjects and patients with pancreatic disease, showed that starch digestion may be impaired in patients with pancreatic disease. However, this occurs only if amylase output is extremely low. The effect of lactase deficiency on lactose absorption was studied in patients with a history suggestive of lactase deficiency. For this purpose a lactose 13CO2 and H2 breath test were compared with lactase activity in a jejunal biopsy. The results showed that the relation between lactase activity in the biopsy and lactose assimilation takes the form of a saturation curve. The 13CO2 breath test was found to be a reliable test for the diagnosis of lactase deficiency. Finally, the effect of acarbose on starch digestion was studied in normal subjects, ileostomy patients, and a fecal incubation system. These experiments showed that acarbose may induce an important degree of starch malabsorption. If administered in high doses, the effect is not only related to inhibition of brush border enzymes, but also to the inhibition of alpha-amylase.