Embryonic diapause in the roe deer is initiated at the blastocyst stage of development and persists for a period of 5 months. In common with other species exhibiting such diapause, quiescence of the roe deer blastocyst is associated with inhibition of endometrial secretory activity. The termination of diapause is characterized by the rapid elongation of the chorionic sac to a length of 30 cm and the simultaneous release of a secretion from the endometrial ducts and glands. The factors responsible for inducing this change in endometrial secretory activity are unknown. Although increases in plasma concentrations of oestrogen and progesterone are observed at the time of termination of diapause, these changes appear to be a consequence rather than a cause of embryonic growth.