Adult female mice of the DDY/S strain were used to study the distribution of PRL or GH cells and the vasculature of the anterior pituitary lobe. Electron microscopy was used to quantify PRL or GH cells in horizontal sections. Most parenchymal cells were either PRL or GH cells, and both types of cells were present in all regions. The densities of PRL cells in the rostral and caudal areas were significantly greater than that of GH cells. The density of GH cells was greater in the anterolateral wings. Thus, the spatial differences in concentrations of PRL and GH cells were reversed. The vasculature was studied with scanning electron microscopy of vascular casts and with stereoscopy of pituitary glands injected with India ink. The adenohypophysis was supplied by long and short portal vessels. The long portal vessels originated from the primary capillary plexus on the median eminence and the upper portion of the pituitary stalk, and they supplied rostral regions of the adenohypophysis. Most of the short portal vessels connected caudal areas of the anterior lobe with the posterior lobe, crossing the surface of the intermediate lobe. The blood in the short portal vessels may flow from the posterior lobe toward the anterior lobe. Thus, within the rostral and caudal areas, which are supplied by long and short portal vessels, respectively, PRL cells predominated; the anterolateral wings where GH cells predominated were far from these regions. These data suggest that the anatomical pattern of the blood supply may account in part for the spatial distribution of PRL and GH cells.