Distributions of embryos and 500-micron diameter microspheres were measured in the cleared oviducts of 32 rabbits at 8 intervals post coitus (pc) and normalized to percentage of isthmic length. By 18 h pc, 46% of the embryos had entered the isthmus and were denuded of the cumulus, while 55% of the microspheres had entered the isthmus. By 24 h pc, all embryos and microspheres were in the isthmus. At 72, 78, and 84 h pc, 6.5, 37, and 93% of embryos and 21, 73, and 95% of microspheres were in the cornu, respectively. The mean positions of embryos and microspheres progressed at approximately 1% of isthmic length per hour between 24 and 72 h pc. Throughout isthmic transport, embryos and microspheres in individual oviducts were tightly grouped and had similar statistical distributions. Although microspheres began to transfer into the uterine horns earlier than embryos, the data suggest that 500-micron microspheres can be used in studies to quantify discrete movements within the oviduct prior to 72 h pc. The data further suggest that opposing forces may be generated by contractile events that keep embryos grouped and possibly control their rate of progress through the oviduct.