1. The intestinal absorption of [ 35 S]thiamine hydrochloride was investigated in healthy subjects, malnourished alcoholics and a patient with resection of the jejunum and ileum. Serum and urinary radioactivity was studied after administration of 10 μCi of [ 35 S]thiamine hydrochloride in 1–50 mg of non-radioactive thiamine hydrochloride. 2. Results suggest that intestinal absorption of thiamine hydrochloride is rate-limited. Though the results provide only indirect information on intestinal transport rates, they are consistent with the Michaelis-Menten relationship used to describe enzyme-substrate reactions. Calculations by this model yielded a V max . of 8·3 ± 2·4 mg and a K m of 12·0 ± 2·4 mg for normal subjects with a significant decrease in V max . in malnourished alcoholics and a patient with resected small intestine. 3. Intestinal absorption and the calculated value of V max . for thiamine hydrochloride is increased in malnourished alcoholics after correction of malnutrition. These findings are consistent with the thesis that this vitamin is absorbed by a saturable mechanism and that the number of effective receptor sites may be reduced by malnutrition or intestinal resection.