Ouabain-sensitive fluid accumulation and ion transport by rabbit blastocysts.

1. By incubating 6‐day post‐coitum (p.c.) blastocysts in medium supplemented with either 20 mM‐sucrose or 10 mM‐KCl it was possible to show that the trophectoderm of the blastocyst prevents the concentration of K in the blastocoele fluid rising to external concentrations. 2. The concentrations of K in the blastocoele fluid are maintained predominantly by leakage of K from the trophoblast cells into the blastocoele and by ouabain‐sensitive transport of K into the trophoblast cells from the blastocoele fluid. 3. Exposure of blastocysts to ouabain on the juxtacoelic, but not abcoelic, surface of the trophectoderm may inhibit blastocoele fluid accumulation.