In an attempt to clarify whether or not glutaraldehyde molecules contribute to the effective osmotic pressure of the fixative solution the ultrastructure of noncultured and in vitro-cultured day 1 3 and 4 rabbit embryos was evaluated. Total osmolarity of the fixative solution (200–800 mosm) was only varied by changing the aldehyde concentration, whereas the vehicle osmolarity (145 mosm) remained unchanged. Optimum preservation in all embryonic stages was obtained when total osmolarity of the fixative solution was 285–340 mosm. Higher (480–800 mosm) or lower (250 mosm) osmolarities of the fixative solution led to alterations mainly in mitochondria and smooth-surfaced endoplasmic reticulum. Shrinkage of cells and condensation of the cytoplasm occurred only occasionally. Compared with early cleavage stage embryos blastocysts were generally more susceptible to hyperton and hypoton fixative solutions. In vitro culture for 24 h per se did not have any influence on the fixation.