Previous studies have shown that a pharmacologic dose of estradiol given at the time of injection of human chorionic gonadotropin (HCG) significantly delays the transport of ova through the rabbit oviduct and that the site of delay is located at the ampullary-isthmic junction. The current experiments investigated the effects of varying the timing of the estrogen injection on ovum transport rates. Fifty-eight rabbits received injections of either Depot-estradiol or crystalline estradiol at various times before or after the injection of HCG. "Tube-locking" resulted when estrogen was administered before or at the time of HCG administration, whereas acceleration occurred when estrogen was given 12 to 24 hours after HCG. The results demonstrate that pharmacologic doses of estrogen exert two effects on ovum transport in the rabbit: (1) delay at the ampullary-isthmic junction and (2) acceleration once that junction has been traversed. The possible application of these results to contraception is discussed.