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Role of age and length of oestrous cycle in alteration of the oocyte and intrauterine environment in the rat

Summary. Zygotes were transferred, on the day of fertilization, from young and old rats with 4- or 6-day oestrous cycles into the ovarian bursa of young recipients with 4-day cycles, and zygotes from young rats with 4-day cycles were transferred into young and old recipients with 4- or 6-day cycles. Young rats with 4-day cycles served as controls for both donors and recipients. An increase in length of cycle or maternal age of donor caused an increase in unfertilized and/or abnormal eggs at the pronuclear stage (nontransferred zygotes). Increased age of donor or length of cycle decreased the implantation rate observed on Day 11 of pregnancy. Likewise, increased age of recipient or length of cycle decreased implantation rate observed on Day 11 of pregnancy. The increase in both age and length of cycle of donor or recipient caused the greatest decline in implantation rate and percentage of normal embryos observed on Day 11 of pregnancy.