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Daily oviducal fluid samples were collected from rabbits for 3 days before and 21 days after ovulation. Pronuclear rabbit embryos collected 15^h r after an ovulatory injection of HCG were cultured in drops of oviducal fluid under paraffin oil for 1 to 9 days to determine whether the fluid would support normal cleavage and expansion of the blastocyst.In oestrous fluid, optimal development was at the four-cell stage and only 8 % of the dividing embryos attained the expanded blastocyst stage. In 8-day oviducal fluid, 88 % became expanding blastocysts, frequently to diameters of 3 mm or more. In fluid from the 13th day of pseudopregnancy to the end of the cycle, embryo development was no better than in oestrous fluid.It is suggested that a component of oviducal fluid, particularly active on Day 2 and Days 8 and 9 after ovulation, significantly enhances the developmental capacity of the preimplantation rabbit embryo. The component(s) is heat labile at 60°C but is stable for at least 9 days when stored at 4°C. This activity is not dependent on the presence of ova in the oviducts, nor does it require interaction of the fluid component(s) with the formed mucin coat of the embryo.

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