The complete requirements for early embryo development in vitro of the ovine and other domestic species remain unknown. Many studies have concentrated on new media, supplementation, and gas atmosphere formulations. A newer approach is to coculture early embryos with different cell types to provide the physico-chemical requirements for their development. In this study, oviduct epithelial (OEC) and dissociated embryo cell (DEC) growth were tested in minimum essential media (MEM) and RPMI. Media were supplemented with fetal calf serum (FCS) and equine derived serum (EDS). Fetal calf serum supported maximum cell confluence in OEC collected on day 3 and day 13 post-estrus. Although at a slower growth rate, DEC developed faster in FCS-supplemented media. Cell growth was slower for EDSsupplemented media in all treatments. As a result, FCS-supplemented media were used to evaluate early embryo growth in various coculture systems. In MEM + OEC, 67% of 1- to 1 0- cell embryos developed to the hatched blastocyst stage (following 8 days of culture). In MEM+DEC, 66% hatched after the same time period. In control treatments (no exogenous cell layers), all embryos degenerated. When early embryo development was compared between St.Croix and Targhee-type breeds in MEM+OEC and RPMI+OEC, no significant differences were observed. The improved results obtained with coculture systems may provide an important method for assessing the viability of embryos following micromanipulation techniques (such as splitting, gene transfer, or following long periods of freezing). The nature of the beneficial action of these coculture systems remains unknown.