Electron microscopy revealed the presence of nonviral nuclear inclusions in human nontumorous as well as adenomatous adenohypophysiocytes, regardless of cell type. Based on ultrastructural appearances, the inclusions have been classified as simple bodies, complex bodies and filamentous aggregates. Many transitional forms were noted between the simple and complex bodies, however, no relationship between the nuclear bodies and filamentous aggregates was found. It can be concluded that the three types of inclusions are normal nuclear constituents. Since no accumulation of these structures was observed in cells associated with enhanced secretion it appears that they are not related to hormonal hyperactivity in the human adenohypophysis.