WNT signalling regulates cell proliferation, differentiation, polarity and organisation. The present study investigated the effects of progesterone (P4) on the endometrial WNT system in relation to blastocyst development and growth in sheep. Ewes received daily intramuscular injections of either corn oil (CO) vehicle or 25 mg P4 from 36 h after mating (Day 0) until hysterectomy on Day 9 or 12. Another group received P4 until Day 8 and 75 mg mifepristone (RU486) from Day 8 to Day 12. Early P4 treatment increased blastocyst growth on Days 9 and 12, whereas no blastocysts were recovered from P4 + RU486-treated ewes. Levels of WNT2 mRNA in the stroma and WNT11 and WNT7A mRNAs in the endometrial luminal epithelia (LE) were reduced in P4 + RU486-treated ewes on Day 9, whereas WNT11 mRNA was reduced in the endometria of both P4- and P4 + RU486-treated ewes on Day 12. On Day 12, WNT2 mRNA was increased in the stroma, WNT7A mRNA was increased in the LE and WNT5A mRNA was increased in the LE and stroma of P4 + RU486- compared with P4-treated ewes. DKK1 mRNA was absent in the endometrial stroma of P4 + RU486-treated ewes. Expression of transcription factor 7 like-2 (TCF7L2) was transiently increased in endometrial epithelia of P4-treated ewes on Day 9, but decreased in these ewes on Day 12. MSX1 mRNA was decreased by P4 treatment on Day 9 and levels of both MSX1 and MSX2 mRNA were higher in P4 + RU486-treated ewes on Day 12. Thus, P4 modulates the endometrial WNT system and elicits a transient decline in selected WNT pathways and signalling components, which is hypothesised to alter tight and adherens junctions, thereby stimulating blastocyst growth and development.