Immunohistochemical (immunoperoxidase) studies were performed on 478 sections from 97 rat pituitary glands with rabbit antisera to unconjugated human synthetic calcitonin, β-endorphin, and/or ACTH-(17-39). Calcitonin-positive cells were present in a majority of the anterior lobes studied, whereas they were present in only a minority of the intermediate lobes. Calcitonin-positive cells were also present in chronically thyroidectomized animals. β-Endorphin-positive cells were uniformly present in the intermediate lobes as were the ACTHpositive cells. In the anterior pituitary lobes, β-endorphin-positive cells were more populous than the ACTH-positive cells, and in general, there was a dissociation of the cellular elements containing β-endorphin, ACTH, and calcitonin. Although it remains possible that there is calcitonin-like immunoreactivity within a precursor molecule that is differentially processed by pituitary cells, these studies are more consistent with the view that immunoreactive calcitonin is present in pituitary cells which are not as yet precisely and consistently related to any identifiable population of hormone-producing cells. (Endocrinology106: 1966, 1980)