A rat bio-assay has been developed in which the tracer ion technetium 99 m (99mTc) was used to monitor absorption from both the antrum and the jejunum of the rat. 2. Evidence has been obtained for the release of a humoral substance from the rat gastric fundus after alkalinization of the mucosa at pH 8, and this fundic factor has been shown to reduce the absorption of the tracer ion 99mTc from both the lumen of the antrum and of the jejunum in the rat. 3. Intravenous infusion of a neutral extract of porcine fundic mucosa, distension of the rat fundus with air or exposure of the fundic mucosa to alkaline buffer all decreased absorption of the tracer ion virtually to the same extent from the lumen of the antrum (P less than 0.001) and to a lesser extent from the lumen of the jejunum (P less than 0.05). This effect does not appear to be due to histamine. 4. Preliminary biochemical data have indicted that the biological activity in the mucosal extract is protein in nature and has a molecular weight of less than 10,000.
