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1. Transepithelial sucrose and urea permeability coefficients were measured in different-aged preimplantation rabbit blastocysts. Sucrose permeability remained constant from 4 to 6 days after fertilization (approximately 1 X 10(-8) cm s-1), but thereafter increased. Urea permeability was very low throughout this period (5 X 10(-7) cm s-1) but a phloretin-sensitive component to urea influx appeared between day 6 and day 7 post coitum (p.c.). 2. Glucose uptake was small (0.2-0.5 nmol cm-2 s-1 and independent of external sodium throughout the preimplantation period. 3. Methionine uptake was 10 times greater than glucose uptake, and was strongly dependent upon the presence of external sodium in 5 and 6 day p.c. blastocysts. The sodium dependence of methionine uptake was lost by the seventh day after fertilization. 4. The transepithelial electrical resistance was always less than 35 omega cm2 in 4 day p.c. blastocysts, and averaged 1758 omega cm2 in 6 day p.c. blastocysts. 5. The transepithelial influx of 140 La averaged 0.46 +/- 0.11 nmol cm-2 h-1 (n = 6) in 4 day p.c. blastocysts. The 5 day p.c. embryos, on the other hand were impermeable to lanthanum (n = 10). 6. These results indicate that the epithelium of the preimplantation rabbit blastocyst undergoes dramatic changes in transport characteristics with development. The blastocyst can be characterized as a 'leaky' transporting epithelium during the early blastocyst period, and as a 'tight' epithelium during the mid to late stages of the preimplantation period.