Histologic examination revealed large ganglion cells within the posterior pituitary of an 80-year-old woman who died of myocardial infarction. Apparently fully mature, the cells were an incidental finding scattered within hyperplastic foci of pars intermedia (PI)-derived cells (basophil invasion) on histologic examination of the pituitary obtained at autopsy. Immunocytochemistry showed staining reactivity for neuron-specific enolase, synaptophysin, alpha subunit of the glycoprotein hormones and beta-endorphin. The presence of these ganglion cells with features similar to those of magnocellular hypothalamic neurons could be considered the result of abnormal migration during the early phase of embryonic life, or differentiation/maturation of neuroblasts, presumed to occur in the embryonic neurohypophysis. Alternatively, transdifferentiation from proliferating PI cells may explain the emergence of neurons; a hypothesis supported by the proximity and shared alpha subunit, and beta-endorphin immunoreactivities of the two cell types.