Estrus, also spelled Oestrus, the period in the sexual cycle of female mammals, except the higher primates, during which they are in heat—i.e., ready to accept a male and to mate.

Estrus suppression can be achieved starting the treatment at least 30 days before onset of the next proestrus and for as long as estrus suppression is desired. Prolonged postponement can be achieved in bitches for up to 2-5 years. Return to estrus averages about 70 days, with a range of 7 to 200 days.