Both the endometrium and the trophoblast secrete a number of proteins. Some of these, e.g. human chorionic gonadotrophin (hCG) from the trophoblast and pregnancy-associated endometrial α2-globulin (α2-PEG) from the endometrium, are secreted into the bloodstream and may act as endocrines. Moreover, they may also function as paracrines, signalling to adjacent tissues. Several models have been developed, by which the paracrine activity of the endometrium and the trophoblast might be examined in vitro. These include, cultures of endometrium and trophoblast, dual chamber superfusion of membranes and dual perfusion of individual placental cotyledons. The purpose of the present study was to examine some aspects of experimental design on the results from these models and consider the factors that might govern the rate of release. A good many trophoblast proteins have been characterised and methods devised for their quantitative measurement, however, the study of endometrial proteins is less advanced. As a first step, a method was developed for the measurement of α2-PEG, the major secretory protein of the secretory endometrium during the first trimester of pregnancy. This thesis describes the development of a competitive ELISA for measuring α2-PEG. This assay was employed for measuring α2-PEG in the medium from decidual cultures. In this thesis, release of protein in vitro has been examined as a mode of basal unstimulated secretion in vivo. The release is compounded of at least two simultaneous processes, biosynthesis and the transport of formed material to the outside of the cell. The concept is put forward that protein release is determined by a dynamic balance between biosynthesis of new protein and diffusion of protein down the concentration gradient between cell cytoplasm and the surrounding medium.