Mature rabbits received norethisterone-3-oxime (NETO) 2 mg/kg/day from day 2 through day 4 of pregnancy by gavage. Control rabbits were given vehicle. All animals were sacrificed on day 5. The oviducts were removed and sections stained with HE. Morphological changes were evaluated. The outer and inner diameters of tubes, and the thickness of circular muscle layers were measured. The layers of the circular muscle cells and the mean thickness occupied by the cytoplasmic mass of the muscle cells were estimated. The fertilized ova flushed out from the oviduct were stained together with the section of ampullary-isthmic junction (AIJ) and isthmus by PAS. The intensity of the staining was observed microscopically and quantified with a microspectrophotometer. NETO treatment led to tortuosity of oviducts showing hyperplasia of epithelial cells and a mucosal edema. The circular muscle layer was thickened but no increase in the number of muscle cells was observed, indicating muscular contraction. Dilatated and congested capillaries appeared in the muscular and subserous layers. All these manifestations were so prominent that the lumen of AIJ and isthmus portion were significantly narrowed as shown by a decreased percentage of inner diameter to outer diameter at AIJ and isthmus. The amount of mucin secreted into the lumen was decreased, but there was an accumulation of mucin in the epithelial cells. The above-mentioned morphological changes lead to the suggestion that the anti-implantational action of NETO is exhibited by its effect on the oviduct in rabbits. Most probably, the action is mainly related to the estrogenic and anti-progestational activity of NETO.