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Pregnant voles react to the presence of unfamiliar males by aborting the offspring they are carrying, entering estrus and mating with these new males. This phenomenon was first described in pine voles by Marks and Schadler (1979). At that time we reported that 84% of females that were four days along in their first pregnancy were induced by strange males to reject their embryos. These findings encouraged us to continue our investigations and this paper reports results on the abortion response caused by strange males in females ten days along in their first pregnancy (Experiment I) and on experienced pine vole mothers that are pregnant and are at the same time nursing a litter (Experiment II).