PRL and GH cells in the adenohypophysis of adult mice were studied with immunohistochemistry and stereological morphometry by light and electron microscopy. We compared findings for males and females. In female mice, most PRL-immunoreactive cells (43% of all parenchymal cells) and most GH-immunoreactive cells (33% of all parenchymal cells) were classical PRL cells (type I PRL cells) or GH cells (type I GH cells), respectively. Besides these classical types of cells, we found PRL- and GH-immunoreactive (type II PRL and GH) cells that contained small (approximately 100-200 nm in diameter) and round secretory granules. However, in female mice type II PRL or GH cells accounted for less than 3% of all parenchymal cells, as was also true for type II GH cells in the males. The percentages of type I and type II PRL cells in male mice were about the same (approximately 10% of all parenchymal cells). Only a few mammosomatotropes (0-0.6% of all parenchymal cells) were found in the mouse adenohypophysis of either sex. All mammosomatotropes were type II cells; none were PRL or GH cells of the classical type. This immunohistochemical study confirmed our previous findings by conventional electron microscopy that there are marked sex differences in the proportions of PRL and GH cells in the adult mouse adenohypophysis; PRL cells are more abundant in the female mice, and GH cells are more abundant in the males.