Prostaglandin F2α. (PGF2α) is luteolytic in the ewe when infused into the ovarian artery (Barrett, Blockey, Brown, Cumming, Goding, Mole & Obst, 1971; McCracken, Baird & Goding, 1971) or the ipsilateral uterine vein (Thorburn & Nicol, 1971). These observations, together with the identification of PGF2α in the uterine venous plasma of anaesthetized ewes towards the end of the oestrous cycle (Bland, Horton & Poyser, 1971; McCracken et al. 1971), strongly suggest that PGF2α is the uterine luteolytic factor in sheep. This paper describes in detail the changes in PGF concentration in utero—ovarian venous plasma of conscious ewes during the oestrous cycle. Polyvinyl catheters (1·5 mm o.d.) were inserted into the right and left utero—ovarian veins via uterine vein branches in two Merino ewes 10 and 11 days after oestrus. Utero—ovarian venous samples were collected every 2–3 h from the day after surgery. Concentrations of PGF