Epidermal growth factor receptor (EGF-R) was studied in Dunning prostatic cancer models using competitive bindings assays and solution hybridization assay. EGF-R-binding capacity and mRNA were demonstrated in a hormone-sensitive R3327 prostatic tumor from both control and castrated animals while no such activity was found in the hormone-independent AT-1 tumors. Castration induced no quantitative changes in the EGF-R. Estrogen treatment induced a significant reduction of the binding capacity of EGF-R and its mRNA. It was concluded that EGF-R is present in the androgen-sensitive Dunning prostatic tumor models (R3327), but that the androgen-insensitive, undifferentiated AT-1 tumor lacks EGF-R expression. Endocrine treatment has no significant effect on the EGF-R in these tumor models.