Preimplantation embryos sired by hamsters without accessory sex glands (ASG) were found to have a higher mortality rate and a slower cleavage rate than those sired by sham-operated males at 72 h post coitum (p.c.). A time-course study of fertilization in vivo was conducted to determine whether this effect was due to delayed fertilization. Ultrastructural morphometry of 48 h embryos was also undertaken to establish the earliest manifestation of developmental anomalies. Compared to sham-operated controls (SH), ablation of all the ASG (TX), or just the ventral prostate (VPX) or ampullary gland (AGX) had no effect on the timing of sperm penetration, extrusion of the second polar body and pronuclear formation. Females mated with AGX males tended to have more polyspermic embryos (9.7%; p < 0.05). The volumes, volume fractions (VV) of the blastomere nuclei, mitochondria and yolk material of the four-cell embryos sired by these same groups of males were assessed using point counting techniques. No difference in the VV of yolk and mitochondria could be observed between groups. However, the SH group did have a significantly larger proportion of the cell occupied by the nucleus (p < 0.05), and the TX group had a higher proportion of the nucleus occupied by nucleoli when compared with the SH group (p < 0.01). Smaller nuclei and larger nucleoli in the TX group was interpreted as an early manifestation of a slower division rate of the blastomeres.