Our objective was to investigate the effect of gram-negative bacterial infection on the ovaries and serum level of P 4 and 17β-E 2 during the preimplantation days of pregnancy in the mouse. We found that lipopolysaccharide alters the serum level of P 4 and E 2 during the preimplantation days of pregnancy and elevates the E 2 /P 4 ratio, which may keep the uterus nonreceptive during the preimplantation days of pregnancy and also not prepare the developing blastocysts for implantation in the mouse. A large infiltration of macrophages in the corpora lutea and appearance of graafian follicles from day 3.5 of pregnancy because of lipopolysaccharide treatment, which reflect a gram-negative bacterial infection, may be responsible for ovarian dysfunction and altered P 4 and E 2 level in serum.