Histochemical staining for carbonic anhydrase (CAH) was demonstrated in the mouse pancreas. Light microscopically, a strong reaction for CAH was found in the B-cell region of the islets, in blood vessels inside and outside the islets, and in centroacinar cells and ductules. In contrast, no distinct reaction was seen in the peripheral A- and D-cell regions of the islets, or in the cytoplasm of the acinar cells. A weak, irregular staining could, however, be observed in some nuclei of the acinar parenchyma. A newly-developed ultrastructural modification disclosed a distinct reaction in a varying number of the secretory granules of the B-cells, whereas a sparse, irregularly-distributed reaction was observed in the nuclei and cytoplasmic ground substance, and in association to the plasma membranes and microvilli of these cells. The CAH reaction was in all portions of the pancreas abolished by acetazolamide. The specificity of these results is discussed. The findings are suggested to indicate presence of a true CAH activity in the B-cells of the mouse pancreas.