Here, we investigated the role of P21-activated kinase 1 (Pak1) signaling in the function of estrogen receptor-alpha (ER-alpha) as assessed by serine 305 (S305) activation and transactivation activity of ER. We found that Pak1 overexpression interfered with the antiestrogenic action of tamoxifen upon the ER transactivation function in hormone-sensitive cells. In addition, tamoxifen stimulation led to up-regulation of ER target genes in breast cancer cells with increased Pak1 expression. Tamoxifen also increased Pak1-ER interaction in tamoxifen-resistant but not in tamoxifen-sensitive cells. Results from the mutational studies discovered a role of ER-S305 phosphorylation in triggering a subsequent phosphorylation of serine 118 (S118), and these effects were further potentiated by tamoxifen treatment. We found that S305 activation-linked ER transactivation function requires a functional S118, and active Pak1 signaling is required for a sustaining S118 phosphorylation of the endogenous ER. All of these events were positively influenced by tamoxifen and thus may contribute toward the loss of antiestrogenic effect of tamoxifen. These findings suggest that Pak1 signaling-dependent activation of ER-S305 leads to an enhanced S118 phosphorylation presumably due to a conformational change, and such structural modifications may participate in the development of tamoxifen resistance.