The study was designed to determine the in vitro effect of bacterial endotoxin – lipopolysaccharide (LPS) – on luteinizing hormone (LH) secretion from anterior pituitary (AP) explants collected from saline-treated ‘healthy’ and LPS-treated ewes in the follicular phase. In the AP explants, the expression of LH-β, gonadotropin-releasing hormone receptor (GnRH-R) and Toll-like receptor 4 (TLR4) genes was also assayed. It was found that explants incubated alone with LPS, as well as together with LPS-binding protein (LBP), abolished the stimulatory effect of GnRH on LH release. LPS added without LBP did not suppress LH-β gene expression in ‘healthy’ APs, however. Moreover, LBP intensified the inhibitory effect of LPS on gene expression in ‘healthy’ APs. These results show that LPS is a potent negative modulator of LH secretion and suggest that its direct action on the pituitary gland could be one of the mechanisms via which an immune/inflammatory challenge inhibits the reproductive process.