The effect of prostaglandin F2 alpha (PGF2 alpha) on blastocyst implantation in spayed rats has been studied. In preliminary experiments, the first implantation sites were observed 8 - 12 hours after a single injection of estrone in ovariectomized and progesterone-conditioned rats. Intraluminal instillation of PGF2 alpha into the right uterine horn 8 - 10 h after the estrone injection increased the number of implantation sites. Even treatment with PGF2 alpha without previous estrone injection induced the first step of blastocyst implantation as shown by uterine dye site reaction (Niagara-blue test). The results are discussed with regard to the possible role of PGF2 alpha in the regulation of the blastocyst implantation processes in the rat.