The cyclic rise in expression of anterior pituitary gonadotropins coincides with the appearance of cells sharing gonadotropic and somatotropic phenotypes. To learn more about possible factors that regulate the origin of this cell type, we studied the time of appearance of cells that co-expressed growth hormone (GH) and gonadotropins and estrogen receptors during the estrous cycle and compared this timing with known changes in regulatory hormones or their receptors. The first event in this cell population is an increase in expression of estrogen receptor (ER)β by GH cells from estrus to metestrus suggesting that estrogen may mediate this early change. Expression of GH mRNA rises rapidly from metestrus to mid-cycle. The rise is seen first in GH cells and then in cells with luteinizing hormone (LH) antigens. These data suggest that, early in the cycle, cells bearing GH and growth hormone releasing hormone (GHRH) receptors begin to produce LH and gonadotropin releasing hormone (GnRH) receptors. Early in proestrus, there is an increase in cells with GH and follicle-stimulating hormone (FSH) suggesting that this set of multipotential cells develops later than GH-LH cells. This fits with earlier studies showing the later rise in expression of FSH mRNA. Collectively these data suggest that the anterior pituitary contains a subset of GH cells that have the capacity to respond to multiple releasing hormones and support more than one system.