The aim of this study was to determine the suitable recipients for transfer of zona-free embryos cultured for 24 hours and possible effects of zona pellucida on the results by comparing the survivals of zona-free and zona-intact embryos. Eight-to sixteen-cell embryos were recovered from oviduct and uterus, and the zona in a part of them was removed in acidified Tyrode's solution. After incubation in M16 medium for 24 hours, 91% of embryos without zona developed to compacted morulae and blastocysts. The birth rates of compacted morulae without zona were 54.6% upon transfer to oviducts of 0.5-day pseudopregnants, and 12.5% upon transfer to uteri of 2.5-day pseudopregnants. The birth rates of blastocysts without zona were 53.8% upon transfer to oviducts of 0.5-day pseudopregnants, and 75% upon transfer to uteri of 2.5-day pseudopregnants. No newborn could be recovered when blastocysts without zona were transferred to the oviduct and uterus of 1.5-day pseudopregnants. When such blastocysts were transfer-red to 3.5-day uterus the birth rate was 26%. There was no significant difference between the results of culture and transfer from embryos with and without zona (p>0.05). These results indicate that the most suitable transfer environment is the oviduct of 0.5-day pseudoregnants for zona free morulae and the uterus of 2.5-day pseudopregnants for zona free blastocysts, The latter can be alternatively transferred to the oviduct of 0.5-day pseudopregnants.
