Altrenogest is used for suppression of estrus.
The mare will experience a predictable occurrence of estrus following drug withdrawal.
This allows a regular cycle during the transition from winter anestrus to the physiological breeding season in mares with active ovaries.
By managing the reproductive cycle, horse owners have more control over breeding procedures and the timing of the arrival of foals.

Altrenogest (Regu-Mate?) is indicated (labeled) to suppress estrus in mares to allow a more predictable occurrence of estrus following withdrawal of the drug.
It is used clinically to assist mares to establish normal cycles during the transitional period from anestrus to the normal breeding season often in conjunction with an artificial photoperiod.
It is more effective in assisting in pregnancy attainment later in the transition period.
Some authors (Squires et al. 1983) suggest selecting mares with considerable follicular activity (mares with one or more follicles 20 mm or greater in size) for treatment during the transitional phase.
Mares that have been in estrus for 10 days or more and have active ovaries are also considered excellent candidates for progestin treatment.
Altrenogest is effective in normally cycling mares for minimizing the necessity for estrus detection, for the synchronization of estrus, and permitting scheduled breeding.
Estrus will ensue 2 – 5 days after treatment is completed and most mares ovulate between 8 – 15 days after withdrawal.
Altrenogest is also effective in suppressing estrus expression in show mares or mares to be raced. Although the drug is labeled as contraindicated during pregnancy, it has been demonstrated to maintain pregnancy in oophorectomized mares and may be of benefit in mares that abort due to sub-therapeutic progestin levels.
The product Matrix? is labeled for synchronization of estrus in sexually mature gilts that have had at least one estrous cycle.
Treatment with altrenogest results in estrus (standing heat) 4-9 days after completion of the 14-day treatment period.
Altrenogest has been used in dogs for luteal insufficiency and as a treatment to prevent premature delivery.