Comparative electronmic roscopic and histochemical examinations on the human and cat gall bladder mucosa give evidence that the epithelial cells of this organ are active in a secretory as well as a resorptive capacity. Secretory granula which probably contain neutral mucins appear in the vicinity of the Golgi apparatus and in the apical cell region. The notched lateral cell membranes point to a participation in the resorption of fluid by the cells.