Pregnant Swiss-Webster mice were treated subcutaneously with 50, 100 or 200 microgram PGF2alpha on day 4, 5 or 6 postcoitum (p.c.). Twelve hours later, the animals were killed by intracardiac perfusion with Karnovsky's fixative, uteri were removed and a transverse central slice from each implantation site was embedded in Araldite. The conceptus was found by thick sectioning. Thin sections were taken of each implantation site and examined at the electron microscopic level. PGF2alpha administered at a dose of 100 microgram per animal on day 4 p.c. altered the normal attachment of the blastocyst to the endometrium, as well as formation of the implantation chamber. Cytology of the epiblast and hypoblast was adversely affected by this change. Dosages of 100 or 200 microgram PGF2alpha per animal adversely affected the apical trophectoderm cell of the blastocyst. Treatment with PGF2alpha on day 4 p.c. at a dose of 50 microgram per animal reduced the decidual reaction around the implantation chamber while treatment on day 5 p.c. at a dose of 50 or 100 microgram per animal was associated with an advanced stage of development. Treatment on day 6 p.c. did not appear to have any influence on implantation or blastocyst development.