Part. I: Prostaglandin F2α (PGF2α) was shown to be ‘the’ luteolysin in the ewe on the following criteria: (1) Hysterectomy and/or separation of the uterine horn from the ovary bearing a corpus luteum led to prolongation of the oestrous cycle, (2) PGF2α shortened the cycle when administered in the midluteal phase, (3) PGF2α was identified in uterine venous blood, (4) a mechanism for the transfer of PGF2α from the uterine vein to the ovarian artery was shown to exist, (5) the quantitative aspects of the secretion transfer mechanism and luteolytic potency of PGF2α were adequate to account for the observed phenomena. Part II: These criteria were applied for evaluation of PGF2α as ‘the’ luteolysin in the following species: cattle, goat, horse, pig, guinea pig, rat, hamster, rabbit, monkey, human. On present knowledge, there appeared to be a variation between species from those in which PGF2α was probably concerned in luteolysis and those in which the evidence was against such a role. Part III: PGF2α was shown to be luteolytic when given by continuous infusion for 3–6 h into the lumen of the uterus in cattle as well as sheep. The minimum effective dose was of the order of 7 g/kg. The application of this finding to artificial insemination programmes was discussed.