Using a surgical approach involving double ligatures and transection, together with subsequent recovery of the eggs, heifers mated at the onset of oestrus have been examined for progression of spermatozoa within the oviducts relative to the time of ovulation; the latter occurred 28-31 hours after the onset of oestrus. Evidence was obtained that spermatozoa competent to penetrate the egg(s) do not pass directly to the site of fertilisation at the isthmic-ampullary junction but instead are largely sequestered before ovulation in the caudal region of the isthmus, possibly for 18-20 hours or more. Thus, 3 of 14 eggs (21%) were fertilised when the oviduct was transected in pre-ovulatory animals 2.0 cm proximal to the utero-tubal junction 16 or more hours after mating compared with 7 of 8 eggs (88%) with a similar post-ovulatory transection 28-36 hours after mating (P less than 0.01). A redistribution of spermatozoa appears to be associated with imminent release of the egg. The distal portion of the oviduct is therefore seen as the functional sperm reservoir--that is the immediate source of viable spermatozoa at the time of ovulation. Parallels are drawn with storage of relatively quiescent sperm cells in the distal portion of the epididymal duct, and procedures of insemination are examined in the light of this storage rôle of the isthmus and the reported incidence of fertilisation in cattle.