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The GH₃ pituitary cell line has been used to investigate the role of the oestrogen receptor (ER) as a modulator of mitogenic signals in tumour cells in the absence of exogenous oestrogen. Using a chemically defined, serum- and oestrogen-free medium, we have demonstrated that the pure steroidal anti-oestrogens ICI 182780 and ICI 164384 are capable of blocking growth by more than 50% after 5 days of culture. Studies with conditioned medium have indicated that the basal growth is due to the secretion of autocrine growth stimulatory substances. Under serum- and oestrogen-free conditions, insulin and IGF-I increased the growth rate of these cells by twofold over a 5-day treatment period, and this effect was also blocked by the anti-oestrogens ICI 182780 and ICI 164384 (50% of maximum inhibition at 0·6 and 6 nM respectively). To explore the potential mechanism by which the ER apparently facilitates the growth factor effects under oestrogen-free conditions, GH₃ cells were transiently transfected with a plasmid reporter containing the vitellogenin oestrogen response element (MTV-ERE-LUC). We have shown that as well as oestradiol (OE₂), insulin and IGF-I induce luciferase activity by between two- and sevenfold (four experiments), and these effects were completely blocked by ICI 182780. In contrast, growth factors and OE₂ were unable to induce luciferase expression when transfections were performed with a plasmid reporter lacking the oestrogen response element. The studies presented here strongly suggest that, in the absence of oestrogen, the ER in these pituitary tumour cells has a role in growth, as peptide factors are able to induce its conversion to a state which is capable of up-regulating the transcription of key growth-promoting genes.