The ER alpha (oestrogen receptor alpha)-derived peptide ER alpha 17p activates rapid signalling events in breast carcinoma cells under steroid-deprived conditions. In the present study, we investigated its effects in ELT3 leiomyoma cells under similar conditions. We show that it activates ERK1/2 (extracellular-signal-regulated kinase 1/2), the G alpha(i) protein, the trans-activation of EGFR (epidermal growth factor receptor) and, finally, cell proliferation. It is partially internalized in cells and induces membrane translocation of beta-arrestins. The activation of ERK1/2 is abolished by the GPR30 (G-protein-coupled receptor 30) antagonist G15 and GPR30 siRNA. When ER alpha is down-regulated by prolonged treatment with E-2 (oestradiol) or specific ER alpha siRNA, the peptide response is blunted. Thus the simultaneous presence of GPR30 and ER alpha is required for the action of ER alpha 17p. In addition, its PLM sequence, which interferes with the formation of the ER alpha-calmodulin complex, appears to be requisite for the phosphorylation of ERK1/2 and cell proliferation. Hence ER alpha 17p is, to our knowledge, the first known peptide targeting ER alpha-GPR30 membrane cross-talk and the subsequent receptor-mediated biological effects.