Intravenously administered secretin (1.5 U per animal) causes an increase of the activities of maltase (EC 3.2.1.20) and sucrase (EC 3.2.1.26) in the mucosa of the upper jejunum of the rat. Lactase (EC 3.2.1.23) is not stimulated. Cholecystokinin at the dose of 1.5 U per animal does not change the activities of these enzymes in the mucosa of the rat jejunum.