In the mouse, a variety of oestrogens and anti-oestrogens given by a single injection on the 4th day of pregnancy inhibit implantation. Local administration to a segment of one horn of the uterus caused inhibition with lower dosage levels than were required by subcutaneous injection, but this was never confined to the segment or horn so treated. In the rat, similar phenomena were seen with dimethylstilboestrol and oestradiol, but the locally effective dose of the former equalled the parenteral dose. No dose-response relationship was found for progesterone or ethyl-19-nortestosterone by either route used.