My hypothesis is that the environment of the preimplantation embryo can affect aspects of pre and post-implantation development, which can then subsequently impinge upon postnatal growth, development and physiology. I have investigated the impact of mouse preimplantation environment on embryo lineage allocation and overall size, and have related this to possible long term effects after embryo transfer. Blastocysts developed in vitro from the 2-cell stage had reduced total cell numbers compared to blastocysts developed in vivo at 96 hours post hCG; however, the ICM:TE ratio was equivalent between these two groups. In vitro culture in either IGF-I (1.7 nM and 8.5 nM) or insulin (170 nM and 850 nM) significantly increased ICM cell numbers and the ICM:TE ratio (p<0.05). Altering the size of the pre-compact embryo so as to half or double the normal number of cells resulted in significantly altered total blastocyst cell numbers (p<0.05), with the ICM:TE ratio increasing as the cell number increased (p<0.05). Aggregated ICMs isolated from early blastocysts produced blastocyst-like structures with significantly increased ICM:TE ratios (p<0.05). Using embryo transfer techniques, I have compared the postnatal development of embryos derived from an in vitro environment (termed 'in vitro' mice) with embryos derived from an in vivo environment (termed 'in vivo' mice). In turn, these two groups have been compared with mice derived from naturally mated and superovulated mothers, which are acting as non-embryo transfer controls. One group of offspring have had their litter size corrected at birth to a mean of 6 (naturally mated (6)' mice), whilst another to 4 ('naturally mated (4)' mice) so as to be comparable with experimental groups involving embryo transfer. No significance differences in birth weight were observed between in vitro and in vivo offspring. However, in vitro offspring were significantly lighter that in vivo offspring in a gender dependent manner at 2 weeks of age (males, p = 0.009) and at 6 and 11 weeks of age (females, p = 0.037 0.035 respectively). In addition, at 4 weeks of age, the in vivo males became significantly lighter when compared to the naturally mated males (p = 0.034). At 10, 12 and 13 weeks of age, male offspring derived from embryos cultured in the presence of 1.7 nM IGF-I became significantly lighter than naturally mated (4) males (p<0.05). Male mice derived from embryos disaggregated at the 8-cell stage became significantly heavier from 12 weeks of age, and remained heavier for the remainder of the 27 week study. At 8, 15 and 21 weeks of age, mice derived from embryos developed in vivo or cultured in vitro had significantly elevated systolic blood pressure when compared to non-embryo transfer controls.