The kinetic parameters (with standard errors) describing glycine uptake by mouse 2-cell embryos and blastocysts were determined by non-linear regression. Uptake at both stages was best described by a combination of a non-saturable component and a single saturable uptake system. During development, the rate constants for both components increased, as would be expected from the known increases in surface area, from 9.4 +/- 3.7 pL per 10 min per embryo and 128 +/- 12 fmol per 10 min per embryo in 2-cell embryos to 38.9 +/- 2.1 pL per 10 min per embryo and 258 +/- 15 fmol per 10 min per embryo in blastocysts. In contrast to earlier reports, there was no change in Km, which was 88 +/- 13 microM in 2-cell embryos and 115 +/- 10 microM in blastocysts. Reducing the external [Na+] from 230 mM increased Km for both stages. This effect on Km appeared to be related to [Na+]-2 or [Na+]-3. Vmax was increased in embryos of both stages by increasing [Na+] from 60 to 100 mM. However, whilst further increases to 400 mM were without major effect on uptake by 2-cell embryos, they inhibited uptake by blastocysts. This may result from osmotic effects on trophectodermal transport in the blastocysts. These results suggest that during development to blastocysts, the gly-system that operates in 2-cell embryos may be modified to a less restricted system with a similar Km and a complex dependence on [Na+].