IN the wild state, the European badger (Meles meles L.) becomes pregnant once a year during a post-partum oestrus in February or March1. Fertilisation is followed, at the blastocyst stage of development, by a prolonged embryonic diapause which ends, when implantation occurs, in December or January. Implantation is thus delayed, for 10 months2 and it may be assumed, in accordance with Fries's hypothesis3, that the resumption of development in the winter is a genetically determined and adaptive phenomenon which results in birth during the spring when conditions for the young are most favourable. This pattern suggests that the time of implantation is related to some environmental factor such as photoperiod or temperature. To test this hypothesis, wild female badgers were captured in their set after fecundation and maintained during summer in conditions of ‘artificial winter’. We report here that in such conditions implantation, gestation and birth can be brought forward 4 months.