Successful implantation of an embryo requires adequate depth of invasion in the endometrium, which depends upon decidualization. The aim of the present study was to elucidate why humans experience spontaneous decidualization and menstruation while most other mammals do not. We established a spontaneous decidualization model in pseudopregnant rats with vitamin E deficiency (VED) to investigate mechanisms associated with spontaneous decidualization. Vaginal smears were used to monitor bleeding while vitamin E levels were analyzed with a commercial vitamin E assay kit. Trypan blue staining was used to observe the implantation site at 5.5 days post-coitum (dpc). Uterine morphology, estradiol (E-2) and progesterone levels, and the anti-oxidation system were evaluated at 5.5, 7.5, and 9.5 dpc. The proportion of rats in the VED group exhibiting endometrial bleeding gradually increased (5.9%, 32.3%, and 50%) over three consecutive cycles of pseudopregnancy. Vitamin E levels in the VED group were markedly lower compared to the control group in both the plasma and uterus, while the level of vitamin E in the liver did not differ between the control and VED groups. Spontaneous decidualization in the VED group was validated by histological examination and immunohistochemistry. At 5.5 dpc, the mean serum E-2 level in the VED group was more than twice that of the control group. The mean total anti-oxidizing capability, catalase level, and glutathione peroxidase activity were significantly reduced in the decidualized portion of the VED group compared to controls, while the malondialdehyde level was also significantly higher in the decidualized portion of the VED group. We hypothesize that the E-2 surge at 5.5 dpc and increasing levels of reactive oxygen species are responsible for spontaneous decidualization in VED rats. (C) 2016 Elsevier Inc. All rights reserved.