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One of the characteristics of senescence in the female golden hamster is a sudden decline in the production of offspring. This occurs after 14 months of age and was originally believed to result from increased fetal resorption (Soder- wall, Kent, Turbyfill & Britenbaker, 1960). Later studies showed that, although fetal resorption was a factor (Blaha, 1964a; Thorneycroft & Soderwall, 1969), the major loss occurred before 5\m=1/2\days ofpregnancy (Connors, Thorpe & Soder- wall, 1972). Although no difference exists in the ovulation rate of young and senescent animals as determined by counting corpora lutea (Thorneycroft & Soderwall, 1969; Connors et al., 1972), nothing is known about the viability of ova from senescent hamsters before implantation. Energy metabolism increases considerably during blastocyst formation in the laboratory mouse and rabbit. The increase in O2 consumption during this stage in the mouse is apparently associated with an increased activity in the Krebs' cycle (Brinster, 1971). In the rabbit, this occurs through an alteration in metabolic pathways; tubai embryos use the pentose phosphate shunt and early blastocysts utilize the Embden-Meyerhof pathway and Krebs' cycle (Fridhandler, 1961). If early embryos from senescent animals should fail to alter their metabolic pathways or if faulty enzyme systems exist, this could result in preimplantation loss. This study was conducted to determine whether succinic dehydrogenase was histochemically similar in preimplantation stages of young and senescent golden hamsters. Young virgin hamsters (3 to 5 months of age) and senescent multiparous hamsters (14 to 17 months of age), judged by vaginal discharge to have cycled 3 to 5 times before use, were mated with young males (3 to 6 months of age) of proven fertility. Stages were timed from the onset of ovulation, which was considered to be 8 hr from the beginning of behavioural oestrus (Harvey, Yanagimachi & Chang, 1961). Preimplantation embryos 2, 2\, 3 and 3\ days of age were obtained by either mincing oviducts or flushing uterine horns. Succinic dehydrogenase was demonstrated histochemically using Nitro-blue tetrazolium salt (NBT) according to the method of Ishida & Chang (1965). The formazans formed as blue granules with this salt are known to coincide with the distribution and activity of the mitochondria. All stages were incubated

PREIMPLANTATION STAGES FROM YOUNG AND SENESCENT GOLDEN HAMSTERS: PRESENCE OF SUCCINIC DEHYDROGENASE AND NON-VIABLE OVA