In Exp. 1, 5-8-cell embryos from superovulated cattle were co-cultured with oviducal tissue suspended in Ham's F10 + 10% fetal calf serum (F10FCS) or in F10FCS alone. After 4 days, the proportion of embryos developing into compact morulae or blastocysts was greater (P less than 0.005) in co-culture (38/82; 46%) than in F10FCS (1/27; 4%). In Exp. 2, a solution of collagenase, trypsin, DNAse and EDTA was used to disperse oviducal tissue, which was then cultured in TCM199 + 10% fetal calf serum (M199FCS) to obtain monolayers. Embryos (1-8 cells) were then co-cultured with monolayers or in M199FCS alone. The proportion of embryos developing into compact morulae and blastocysts after 4-5 days was higher (P less than 0.005) in co-culture (15/34; 43%) than in M199FCS (1/37; 3%); mean numbers of cells/embryo were also higher (P less than 0.001) (27.70; range 2-82 in co-culture; 8.83; range 2-18 in M199FCS). In Exp. 3, embryos obtained from in-vitro maturation and fertilization were used to compare development between co-culture and medium conditioned by oviducal tissue. Initial cleavage rate (no. embryos greater than 1 cell/total) was 76% (611/807) and did not differ among treatments. After 5 days, the proportion cleaving to greater than 16 cells was higher (P less than 0.005) in co-culture (71/203; 35%) and conditioned medium (48/205; 23%) compared to M199FCS (14/203; 7%). Similarly, the proportion developing into compact morulae and blastocysts was greater (P less than 0.005) in co-culture (44/203; 22%) and conditioned medium (46/205; 22%) than in M199FCS (7/203; 3%).(ABSTRACT TRUNCATED AT 250 WORDS)