The present study was undertaken to investigate the developing mechanism and its biological significance of the decidua. Unilateral salpingeal ligation was performed on mature female Sprogue-Dawley rats. Mating was proceeded. On the 3rd, 5th, 7th, and 9th days of the pregnancy, morphological changes of both ligated (non-pregnant horn) and non-ligated (pregnant horn) salpinges were observed. Gradual change of the decidualization was observed already by the 5th day of the pregnancy in the non-ligated horn, but was not in ligated. Marked change of the decidualization was observed by the 7th and 9th days of pregnancy in the non-ligated horn, but little change was in the ligated. The results suggest that decidualization is contributed not only by endocrinological factors but also obviously by some factors of the blastocyst.