The influence of medroxyprogresterone acetate impregnated intravaginal sponges and 300 IU pregnant mare serum (PMSG) , the latter injected at different times before sponge withdrawal, on the length of oestrus, the interval from sponge withdrawal to the commencement of oestrus and the time and number of ovulations was investigated. The project was carried out during the breeding and non breeding season. PMSG shortened the interval from sponge withdrawal to the commencement of oestrus and to ovulation. Irrespective of the season, artificial insemination is recommended at 36, 48 and 60 h after sponge withdrawal without the predetermination of oestrus by teaser rams.