In previous papers we have demonstrated that sialoadenectomy inhibited MPA-induced mammary tumorigenesis in BALB/c mice. To further explore the role of EGF in this experimental model, we evaluated its effects on mammary glands of sialoadenectomized (sialox) MPA-treated female mice and on tumor growth. MPA-treated sialox mice were injected s.c. (n = 3) or not (n = 6) with 5 microg EGF every 36 h for 45 days; MPA-treated sham-operated mice were used as controls (n = 6). Mammary glands from sialox MPA-treated mice are considerably less developed as compared with sham-operated animals. The exogenous administration of EGF restores the usual MPA-induced growth pattern of the glands, thus confirming a role for EGF either in mediating or cooperating with MPA in inducing the mammary architectural changes observed in MPA-treated mice. On the other hand, primary cultures of progestin-dependent (PD) ductal mammary adenocarcinoma in vivo tumor lines and of lobular progestin-independent (PI) tumor lines were used to evaluate the effect of EGF on tumor growth. In vitro EGF was found to stimulate cell proliferation of lobular PI tumor cells and of fibroblastic cells from both types of tumors at concentrations higher than 0.1-0.5 ng/ml and in the presence of 1-5% of charcoal-stripped fetal calf serum. Conversely, no proliferative effects were observed in ductal PD cells under the same experimental conditions, regardless of the presence of 10 nM MPA. It can be concluded that although EGF plays an important role in MPA-induced mammary carcinogenesis, it is not necessary in PD tumor growth.