Female brown marsupial mice were mated and changes in the number and distribution of spermatozoa were assessed in several regions of the reproductive tract at 1, 2, 3, 7, 10, 14 and 18 days after mating. Approximately 40 x 10(3) spermatozoa/side were present in the female reproductive tract between Days 1 and 7 after a single mating. This had decreased (to approximately 9 x 10(3) spermatozoa/side) by Days 10 and 14 after mating; by Day 18 no spermatozoa were recovered. The maximum number of spermatozoa recorded in a female tract was approximately 72 x 10(3) spermatozoa/side (Day 5 female, death in laboratory) and the minimum recorded was approximately 2 x 10(3) spermatozoa/side on Day 2 after mating. Between Days 1 and 7 after mating most spermatozoa were located in the uterus and lower isthmus (isthmus 1) and spermatozoa were rarely found in the lateral vaginae. By 24 h after mating most spermatozoa (approximately 60%) were found in isthmus 1, but approximately 35% were still present in the uterus. Histological observations of the lower isthmus at this time showed that large numbers of spermatozoa were present in both the lumen of the duct and the sperm storage crypts which are located in this region. By Day 7 after mating approximately 91% of all spermatozoa in the female tract were in isthmus 1, most of these being confined to the sperm storage crypts. On Days 10 and 14 after mating almost all spermatozoa in the tract were in the crypt regions of isthmus 1 and on Day 18 degenerating spermatozoa were observed. No special orientation or association of spermatozoa in relation to crypt cells was observed. These results show that, although the number of spermatozoa inseminated is low by mammalian standards sperm transport in this species is extremely efficient and a large proportion of spermatozoa reaches the isthmus before ovulation (approximately 1 in 1 to 1 in 7). Several observations may explain the remarkable success of these low numbers of spermatozoa, including specializations of the reproductive tract which may have a directing effect on sperm movement and the special relationship which exists between spermatozoa and the oviducal environment which results in viable sperm storage. Recent observations suggest that an unusual sinusoidal mode of progressive motility observed in this species, may also influence the success of the low numbers of ejaculated spermatozoa.