The activities of soluble (S-COMT) and membrane-bound (MB-COMT) forms of catechol-O-methyltransferase in different regions of human and rat small intestine were determined. S-COMT was the predominant form both in the mucous membrane and in the muscle layers. The activity of S-COMT was more than twice as high in the mucous membrane as in the muscle layer. The activity of MB-COMT was almost equal in the different regions of the gut as well as in the mucous membrane and muscle layers. However, the ratio of the soluble form to the membrane-bound form was close to one in the human muscle layers of the jejunum and ileum.