In this article I shall be discussing our state of knowledge with respect to the composition and function of tubal and uterine secretions with particular reference to the possibilities for contraceptive attack. Before considering the secretions themselves it is important to establish the type of contraceptive we are hoping to develop as a result of these studies. One possible approach might be to search for specific proteins or glycoproteins in the genital tract secretions which are essential for the continuation of early embryonic development and which might, therefore, be used as targets for the immunological suppression of fertility. In both the tubal (Moghissi, 1970) and uterine (Roberts, Parker & Henderson, 1976; Maathuis & Aitken, 1978a, b) secretions of the human genital tract biochemical evidence exists for the presence of nonserum, possibly unique, proteins (Pl. 1, Fig. 1). The specificity of these proteins has not yet been established immunologically, however, and we may do well to remember that the protein "uteroglobin" (Krishnan & Daniel, 1967; Beier, 1968) was thought to be a specific component of rabbit uterine secretions for a decade until it was also discovered in the lungs, tubal secretions and seminal plasma of this species (Noske & Feigelson, 1976). Our uncertainty concerning the specificity of uterine and tubal proteins and the attendant hazards of autoimmune disease (Tung, 1976) should perhaps discourage us from this approach. An alternative route towards the immunological suppression of fertility might be to search for specific embryonic factors which are responsible for regulating the synthesis or release of critical components in the oviductal or uterine secretions. The advantage of this approach is that the target antigen is of embryonic origin and would, therefore, only be exposed to the maternal immune system for a limited period of time. The specificity of the embryonic factors involved would still have to be demonstrated however. Alternatively we might try to regulate the synthesis or release of critical secretory components by pharmacological means, using, for example, anti-oestrogens or antiprogestagens. Common to both the immunological and pharmacological approaches is the need to identify processes in early development in which tubal or uterine secretions play an important part. In the following sections I shall consider the possible roles played by these secretions and assemble the evidence for an embryonic or pharmacological influence on their composition.