The motility and membrane character of rabbit spermatozoa have been studied during transport in the female rabbit reproductive tract. A phase of rapid passive transport has been identified by recovery of spermatozoa from the oviduct ampulla and from the surfaces of the fimbria and ovary within 1 minute after mating. Rapid transport is an asymmetric phenomenon occurring predominately in the left oviduct and it ends within a few minutes after mating. The vast majority of spermatozoa recovered from the upper oviduct at 1 and 15 minutes post coitum are non-motile and have undergone membrane changes seen as disrupted or lifted acrosomal caps. These spermatozoa are cleared to the peritoneal cavity within a few hours after mating and they play no direct role in fertilization. Spermatozoa transported to the oviducts subsequently are confined to the lower 2 cm of the oviduct isthmus prior to ovulation. Isthmic spermatozoa have markedly reduced flagellar activity - low amplitude and frequency of beat, as compared with uterine spermatozoa which are the most actively motile spermatozoa in the female tract. Sperm recovery assays after artificial insemination, with and without an ovulation inducing stimulus, show that ovulation must occur if significant numbers of spermatozoa are to enter the lower ampulla of the oviduct - the site of fertilization. Ovulation per se is not, however, a sufficient cause of sperm ascent to the ampulla. Delayed mating tests show that isthmic spermatozoa do not reach the site of fertilization after ovulation if they have been in the female tract for 4 hours or less. Rabbit spermatozoa must undergo time-dependent changes in the female tract - presumably within the oviduct isthmus - before they can traverse the upper levels of the oviduct isthmus and enter the ampulla. BIBLIOGRAPHIC REFERENCES: J.W. Overstreet and G.W. Cooper. 1976. Rabbit sperm do not survive rapid transport through the female reproductive tract. Society for the Study of Reproduction, Ninth Annual Meeting, August, 1976: Abstract #9, p. 20. G.W. Cooper and R.J. Young. 1977. Mammalian sperm are dissociated into heads and tails by primary amines: Demonstration of intermolecular covalent bonds between the nuclear membranes and basal plate of the tail connecting piece. Anat. Rec., in press.