As a mare enters estrus and prepares for breeding, her levels of estrogen rise. After she ovulates, estrogen drops and is replaced with an increasing amount of progesterone. Progesterone prepares her uterus for pregnancy and prevents abortion of the fetus. Roughly 14 to 16 days after ovulation, if the mare is not pregnant, her progesterone levels decline and the cycle starts over.

Regu-Mate® is a synthetic form of progesterone called progestin. When given daily for up to 15 consecutive days, it reliably keeps mares from entering estrus. When use of Regu-Mate is discontinued, mares will return to estrus, or come into heat, within 4 to 5 days.