The study examined whether co-culture with oviductal epithelial cells was of benefit to ovine in vitro fertilization ( IVF) and embryo culture procedures utilizing ·a well charac- terized culture system based on a synthetic oviductal fluid medium (SOFM) supple- mented with serum in a 90% N2, 5% 0 2, 5% C02, atmosphere at 38.6°C. Two experiments were carried out. In Experiment 1, comparison was made between the frequency of fertil- ization and development of in vitro matured ( IVM) oocytes cultured in the absence (Group 1) or presence of oviductal cells for a 24 h (Group 2), 48 h (Group 3) or 96 h (Group 4) period post insemination. In Experiment 2, comparison was made between the develop- ment of IVM oocytes fertilized and cultured in vitro for 7. 5 days in the absence or presence of oviductal cells with IVM oocytes which had been fertilized in vitro for 20 h in the pres- ence of oviductal cells and then transferred to the oviducts of a recipient ewe, 30 h post oestrus, for a 6.5 day period of in vivo culture. Similar rates of fertilization ( 54-58%) and blastocyst development from cleaving zygotes ( 48-69%) were achieved in both experi- ments in vitro with no evident benefit of including oviductal cells. In fact, fewer (P= 0 .02) blastocysts developed from cleaved embryos when co-cultured for the 96 h period (Group 4). Whilst the blastocyst development rates obtained in vitro ( 45-58%) were similar to, or higher than (P=0.03 ), those obtained in vivo ( 43%), more than 50% of in vitro cul- tured embryos in both experiments showed evidence of fragmentation and/or irregular cleavage as well as a lack of firm compaction at the morula stage. Also the blastocysts that had not hatched, or hatched by 7.5 days of in vitro culture had significantly fewer cells than those cultured in vivo (P