Three isoinhibitors of bovine pancreatic trypsin inhibitor (BPTI) have been identified and isolated from bovine pituitary gland. The results of the purification process by affinity chromatography on immobilized trypsin, the electrophoretic mobility in non-denaturing conditions, the antiproteolytic activity and the immunochemical reactions indicate that these inhibitors correspond to those previously isolated from bovine spleen and lung. In addition, immunohistochemical experiments show that the isoinhibitors and BPTI are exclusively localized in the mast cells, and not in the endocrine cells, of the pars intermedia and posterior lobe (neurohypophysis) of the pituitary gland. The physiological implications of these findings are discussed.