The results presented in this paper indicate that delayed implantation in the roe deer is due to a lack of certain essential factors which are needed to induce and support the process of embryonic growth. These factors are eventually supplied, in the first weeks of January, as a secretion emanating from the endometrial glands. This secretion contains uterine-specific and serum proteins, about twenty free amino acids, protein-bound glucose and galactose and, rather surprisingly, a free ketose which appears to be fructose. Elongation of the roe deer blastocyst is also correlated with a rise in the concentration of plasma oestrogens, an endocrine change that may stimulate the endometrial glands into secretory activity. However, since simultaneous changes were not observed in the ovaries, the elevated oestrogen levels may be a consequence rather than a cause of embryonic growth.