Oestradiol benzoate (ODB) injection of ewes isolated from rams for three months during the anoestrous season resulted in a significantly greater (P<0,001) incidence of oestrus than in anoestrous ewes not isolated from rams. Mating was initiated after a shorter latent period in the isolated ewes than in those continuously with rams. In a second experiment 44 of the 84 ewes were ovariectomized one month prior to the commencement of either isolation from or continuous association with rams. Five months after the ovariectomies, all ewes received progesterone for three; six or twelve days prior to ODB injection. The total number of ewes exhibiting oestrus and the number which mated within 40 h after ODB injection were significantly higher among the isolated ewes than amongst those continuously with rams. Significantly more ewes exhibited oestrus after 6 or 12 d of progesterone than where progesterone was administered for 3 d. Although the latency to oestrus was shorter in the isolated ewes than in those not isolated, the difference was significant only in the ewes not ovariectomized. The presence or absence of ovaries did not influence the results significantly.