Summary. When pre-implantation embryos were transferred to the oviducts of pseudopregnant recipient females, on the 1st day post coitum, the percentage of females becoming pregnant was greater when a very small inoculum volume was used. The number of eggs injected per oviduct, in the range from two to ten, did not significantly affect the percentage developing into foetuses. The pregnancy rate was lower for one-cell than for two-cell and eight-cell eggs, morulae and blastocysts, but the percentage of eggs developing into live foetuses in those females which became pregnant was similar for all stages. Blastocysts transferred to the oviducts of recipients on the first day p.c. passed down the oviduct at a rate similar to that of the recipients' own unfertilized eggs. Removal of the zona pellucida with pronase prevented almost entirely the development of eight-cell eggs to foetuses after transfer to oviducts of recipients on the 1st day p.c., although development in vitro was not affected. Removal of the zona did not reduce the proportion of blastocysts developing successfully, after transfer to either oviduct or uterus. It was concluded that the zona pellucida is necessary for the transit of cleavage stages through the oviduct in normal pregnancy. The rate of transmigration between uterine horns following transfer to the oviduct was very low (0·4%).