Response of ewes to a single injection of an analogue of PGF-2α given at different stages of the oestrous cycle

Six groups of 4 ewes were given a single i.m. injection of 100 microgram ICI 80,996 on Days 3, 5, 8, 11, 14 or 16 of the oestrous cycle. On day 3 treatment was only partly effective in causing luteal regression, with only 2 of the 4 treated ewes returning to oestrus. Injection had no apparent effect when given on Day 16, at a time when luteal regression had already started naturally. However, treatment between Days 5 and 14 inclusive of the oestrous cycle evoked a rapid fall in progesterone concentrations to basal values of less than 0.5 ng/ml within 24 h with a high degree of synchrony in the return of oestrus (37.7 +/- 1.6 h) and the timing of the LH peak (45.3 +/- 3.2 h) after injection. We conclude that the corpus luteum of the ewe is responsive to ICI 80,996 only between Days 5 and 14 of the oestrous cycle.