Summary. The technique of post-coital transection of the oviducts from the uterus at various intervals after mating has been used to establish how soon a population of spermatozoa competent to fertilise the egg(s) first appears in the oviduct of the ewe. Fertilised eggs were not found as a sequel to transection at 4 or 6 hrs after mating, whereas the incidence of fertilisation irt a small series of ewes was 30 p. 100 following transection at 8 hrs and 100 p. 100 at 10 hrs. The mean number of spermatozoa associated with the zona pellucida at these later times increased from 2 to 13.4. The results are interpreted as indicating only a gradual progression of viable spermatozoa from the cervix to the oviduct, although the possible contribution of a rapid phase of sperm transport to the events of fertilisation is discussed. Finally, the isthmus rather than the cervix is reasoned to act as the functional sperm reservoir at the time of ovulation, with some form of peri-ovulatory programming of sperm release from this region of the tract. ' Introduction.