Prepubertal ovariectomy can dramatically inhibit mammary development, but the mechanism of inhibition is not well characterized. Holstein heifers were ovariectomized (OVX) or sham operated but left intact (INT) at d 40 and then sacrificed at d 55, 70, 85, 100, 130, or 160 to provide tissues for histologic analysis of cell proliferation. Our histologic analyses unexpectedly revealed a pronounced effect of ovariectomy on myoepithelial cell development. Myoepithelial cells were identified on the basis of location, morphology, and immunohistochemical staining for α-smooth muscle actin (SMA). Vascular smooth muscle staining served as an internal positive control for all immunohistochemical analyses. Mammary tissues from d 40 heifers had an abundance of SMA+ cells associated with the ductal parenchyma. In INT heifers, the frequency of SMA+ cells decreased as development progressed. Only a limited number of isolated SMA+ cells were observed in d 70 to d 160 INT heifers. In OVX heifers, SMA+ cells were abundant, had elongated morphology, and frequently stained more intensively than vascular smooth muscle cells. The intense SMA staining and altered morphology was most prominent in older heifers. Limited analysis of gene expression revealed that maspin, a protease inhibitor expressed by myoepithelial cells, was expressed in parenchyma from both INT and OVX heifers. Our hypothesis is that ovarian secretions stimulate epithelial proliferation, and block myoepithelial differentiation. Myoepithelial cells are known to limit parenchymal cell proliferation. Ovariectomy may thus remove an estrogenic growth stimulus and permit the emergence of inhibitory cell populations that further limit parenchymal expansion. Our observation has important implications for control mechanisms that regulate parenchymal development.