The development and quality of ovine zygotes matured and fertilized in vitro were compared after coculture with oviductal cells (CZB–199 system) and culture in synthetic oviduct fluid medium without cells (SOF system). The effect of two oxygen concentrations (5% and 20%) on the development of ovine zygotes in SOF medium was also studied. More ovine zygotes reached the blastocyst stage when culture in SOF medium was performed in 5% O ₂ rather than 20% O ₂ . A greater number of blastocysts was obtained after culture in the SOF system than coculture in the CZB–199 system. Proportions of grade I (excellent), II (good), III (fair) and IV (poor) blastocysts did not differ significantly between the SOF and CZB–199 systems. Histological examination of hatched blastocysts revealed a superiority of the SOF system for the following: a greater number of total and trophoblastic cells in grade I and II blastocysts; more endodermic cells in grade I blastocysts, higher mitotic index in the inner cell mass of grade II blastocysts and in total and trophoblastic cells of grade I, II and III blastocysts; more grade III blastocysts with mitosis in the inner cell mass; and a lower pyknotic index in the inner cell mass of grade I, II and III blastocysts. Culture in the SOF system improved the rate and quality of blastocysts in comparison with the CZB–199 system. Furthermore, culture in SOF medium with 5% O ₂ provided more blastocysts than culture in the presence of 20% O ₂ .