Oxytocin administration prolongs luteal function in cyclic mares.

Recent evidence indicates that, in mares, as in the domestic ruminants, oxytocin and its endometrial receptor play important roles in stimulating the pulsatile releases of prostaglandin F2 alpha from the endometrium that effect luteolysis. In the present experiment, continuous administration of oxytocin by subcutaneous minipump to five mares during days 8-20 after ovulation abolished luteolysis in four of them, while all four of the control mares infused similarly with saline underwent luteolysis at the expected time. When oxytocin administration began on day 10, instead of on day 8, after ovulation luteolysis occurred rapidly in two of the five treated mares, indicating that the development of oxytocin responsiveness begins on or about day 10 of dioestrus in cyclic mares.