The objective of the present immunohistochemical study was to determine whether the close spatial relationship between hormone-producing cells as described in rats also exists in the mouse adenohypophysis. In both immature and mature mice, GH cells were the only cell type that had a round shape throughout the pars distalis. All other types of secretory cells had angular or irregular shapes and were closely apposed to round GH cells. Thus, between GH and ACTH cells the same intimate relationship pertains as in rats. Unlike in rats, however, the juxtaposition of LH and Prl cells was observed only occasionally in mature female mice. The salient features of the mouse adenohypophysis were that most LH cells closely surrounded GH cells. These findings show that the cytoarchitectural interrelationship between adenohypophysial cells of mice differs from that of rats.