The effect of different sperm doses on the conception rate has been studied by Salamon (1962), Gancev (1963) and Kareta, Osikowski & Wierzbowski (1966), but without relation to a possible effect of time of insemination. Jones, Martin & Lapwood (1969) and Entwistle & Martin (1972) studied the effect of restricted sperm doses (100 × 106 and 50 × 106 spermatozoa) and, by relating the results to vaginal mucus score, found that the highest conception rate occurred in ewes with a copious, clear or cloudy mucus at the time of insemination. On the other hand, in recent work in this laboratory (Amir & Schindler, 1972), it was shown that with the use of 300 to 500 × 106 freshly ejaculated spermatozoa, single inseminations of ewes were of uniform and high efficiency almost throughout oestrus. The present study was undertaken in order to investigate the possibility that the time
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