REPRODUCTIVE senescence in the female golden hamster occurs after 14 months of age and is characterised by a significant decline in the production of offspring1. Recent studies in our laboratory concerned with the cause(s) of this decline have revealed large numbers of non-viable ova in aged hamsters before implantation2 and a developmental delay of approximately 12 h in preimplantation and implantation stages of embryos from senescent hamsters when compared with young females at the same hour of pregnancy3,4. Since the elapsed time for the union of gametes is important for normal fertilisation and development we examined ova for the onset of fertilisation in young and aged hamsters.