Prostaglandin F2α (PGF2α) has been shown to be luteolytic in sheep when infused into the ovarian artery in situ (Thorburn & Nicol, 1971) or when autotransplanted to the neck (McCracken, Glew & Scaramuzzi, 1970). It was originally suggested that PGF2α might induce luteal regression by causing venoconstriction in the utero-ovarian vein and hence an alteration in ovarian blood flow (Pharriss & Wyngarden, 1969). Variable changes in total ovarian blood flow have been described after intra-arterial infusion of PGF2α in sheep (McCracken et al. 1970; Chamley, Buckmaster, Cain, Cerini, Cerini, Cumming & Goding, 1972). Prostaglandin F2α was infused into the ovary through the ovarian artery at a dose of 40 μg/h in nine experiments in four ewes with autotransplanted ovaries. Ovarian blood flow and progesterone secretion rate were measured, as previously described (Collett, Land & Baird, 1973), at intervals during and immediately after a 4 h infusion (Fig. 1).