Between the 1-cell zygote and the early blastocyst stage of mouse embryos the net rate of uptake of methionine increased, the internal pool became progressively more expanded and less easily reached steady state, and the specificity of competitor amino acids changed. Sodium-dependent transport was first detected in compacted morulae (16--32-cell stage). Uptake of [14C]methylaminoisobutyric acid was detectable in blastocysts but not in unfertilized eggs. Efflux of methionine by an exchange transport system was detectable at all stages, but in intact blastocysts much higher external concentrations were required to activate exchange transport. An exchange system with properties similar to that operating at cleavage stages was exposed when blastocysts were collapsed with cytochalasin D. Since this exchange system was not detectable in isolated inner cell masses, it may be confined to the juxtacoelic surface of trophectoderm cells.