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* Department of Anatomy, University of Texas Medical Branch, Galveston, Texas 77550, U.S.A. and The Worcester Foundation for Experimental Biology, Shrewsbury, Massachusetts 01545, U.S.A.

(Received 5 December 1977)

Fertilization is delayed approximately 2–5 h in the majority of ova recovered from aged hamsters (14–17 months old), compared with younger (3–5 months old) animals (Parkening & Soderwall, 1975). This delay may explain the relatively large percentage (40%) of ova which is incapable of developing to the implantation stage in this species (Parkening & Soderwall, 1975). Blaha & Leavitt (1974) have shown that there is a wider variation in the peripheral plasma concentration of progesterone at the time of fertilization in aged than in younger hamsters. A hormonal imbalance in the senescent female hamster may disturb normal gamete interactions, thus causing the delay in fertilization. In order to determine the significance of hormonal levels with respect to fertilization, the concentrations of progestogens,