In bank voles blastocysts arising from mating immediately after parturition do not implant in used zones of the preceding pregnancy but in the regions between them. However, if mating is postponed until 12 or more days after parturition blastocysts do implant in used zones of the previous pregnancy. Implantations also occur at zones used two pregnancies previously. It seems likely that the absence of implantations at recently used zones of the uterus is determined by the uterus rather than blastocysts.

Summary. Laboratory bred bank voles of different age and reproductive status were bilaterally ovariectomized, treated with exogenous oestrogen or progesterone or both, and given an artificial decidual stimulus (crushing with artery forceps). An hormonal regimen of oestrogen (100 ng) followed by progesterone (1 mg) and small amounts of oestrogen (10 ng) was necessary for maximal development of the decidual cell reaction (DCR). A DCR did not develop if the animals were treated with only oestrogen or progesterone. Middle-aged virgin females 5\p=n-\7months old had larger responses than did young virgin females 2\p=n-\3months old, and parous females, 15\p=n-\19months old and having had at least 10 litters, had a response similar to that of virgin females of about the same age. The size of the DCR in old parous females was inversely related to the time elapsing since the last litter. It is concluded that in its hormonal requirements for a successful DCR the bank vole is similar to the mouse. The results also suggest that the increase in fertility of bank voles up to the age of about 100 days may in part be explained by increased response of the endometrium to blastocysts. The reduction in size of the DCR in old animals is a consequence of ageing rather than of repeated pregnancies.