The influence of medroxyprogesterone acetate impregnated intravaginal sponges and pregnant mare serum (PMSG) on the length of oestrus, the length of the interval from sponge withdrawal to the beginning of oestrus, the length of the interval from the beginning of oestrus to ovulation and the number of ovulations per ewe was investigated. Barren and post partum ewes were used during both the breeding and non-breeding seasons. PMSG had no effect on the length of oestrus. but shortened the period from sponge withdrawal to the beginning of oestrus, advanced the time of ovulation in relation to both the beginning of oestrus and sponge withdrawal, and increased the number of ovulations per ewe. The time interval from sponge withdrawal to the beginning of oestrus was found to be shorter, and the length of oestrus longer in the normal breeding as compared to the non-breeding season. The number of ovulations per ewe, and the interval from sponge withdrawal to ovulation were not subject to seasonal influences: and no differences were apparent between barren and post partum ewes. Without the predetermination of the beginning of oestrus by teaser rams, artificial insemination at 36, 48 and 60 hr after sponge withdrawal is recommended irrespective of the season during which synchronization is applied.