An effect of Tamoxifen (I.C.I. 46,474) on the surface coat of the late preimplantation mouse blastocyst.

The oral adminitration of Tamoxifen on Days 2,3 and 4 after mating, or on Day 2 only, was found to prevent the oestrogen-dependent surface-coat change of the late preimplantation blastocyst in the mouse. It is suggested that his is an antioestrogenic manifestation and that ist is, at least in part, responsible for the drug's known antifertility effect.