In endometriosis, peritoneal macrophages (PMs) showed higher expression of ERβ compared to those from women without endometriosis. Pretreating PMs with ERB-041 significantly inhibited LPS-induced iNOS expression and NF-κB activation by preventing its nuclear translocation. The ERKs (extracellular signal-regulated kinases) pathway contributed to LPS-induced iNOS production and was not suppressed by ER activation./The ERβ agonist’s inhibitory effect on LPS-induced iNOS production in EMS PMs is likely mediated through the suppression of the NF-κB signaling pathway, rather than the ERKs pathway. The gene discussed is NOS2; the disease is endometriosis.