Abstract: Levonorgestrel oxime (LNGO) was administered intragastrically to pregnant rats at daily doses of 5, 7.5 or 10 mg/kg on days 1～4 of pregnancy and at daily doses of 40 or 60 mg/kg on days 6～8 of pregnancy. LNGO showed significant anti-implantation and terminating early pregnancy effects.Levonorgestrel (LNG) was administered intragastrically to pregnant rats at daily doses of 10, 40 or 80 mg/kg on days 1～4 of pregnancy and at daily doses of 40 or 80 mg/kg on days 6～8 of pregnancy. LNG showed no obvious anti-implantation and terminating early pregnancy effects.LNGO was administered intragastrically to pregnant rats on days 1～4 of pregnancy at daily dose of 10 mg/kg. Light microscopic examination of the endometrium showed inhibition of hyperplasia and decidulization of stromal cells. Electron microscopic examination of the endometrium showed regressed microvilli, degenerated organelles and interrupted basal lamina in epithelial cells and dilated perinuclear cisternae in stromal cells (Fig 1: A, B, C).LNG was administered intragastrically to pregant rats at daily dose of 40 mg/kg on days 1～4 of pregnancy. Light and electron microscopic examination of the endometrium showed no morphological changes.LNG was injected directly into the right uterine horn of the rat at a Single dose of 3 or 30 mg each horn on the estrons day(the left horn was used as control). Light and electron microscopic examination of the endometrium showed no morphological changes.Human trophoblastic cells were cultured in media containing LNGO at concentrations of 50 μg/ml or 100 μg/ml and the cells were damaged (Fig 2: A, B, C). If human trephoblastic cells were cultured in media containing LNG at concentratious of 50 μg/ml or 100 μg/ml, no apparent effect was observed.