The lenses and head mesenchyme of two 11-gestational day trisomy 1 mouse embryos and a normal littermate were examined using transmission electron microscopy (TEM). One trisomic embryo had a small lens with a lens stalk; the other was aphakic. The resolution available with TEM allowed detailed evaluation of cell organelles, spatial relationships and the intra- and extracellular structural environment of the lens and head mesenchyme in normal and aneuploid embryos. Differences in fine structure between normal and trisomic lenses included (1) type of epithelium, (2) size of intercellular lacunae, and (3) activity of Golgi complexes. Differences in mesenchyme included (1) the number and size of cytoplasmic extensions, (2) shape of the cells, and (3) reduced endotheliomesenchymal interactions.