The uterus is composed of heterogeneous cell types that undergo cyclic synchronized waves of proliferation and differentiation in response to the rise and fall of ovarian oestrogen and progesterone. The spatial and temporal diversity in cellular responses to ovarian hormones within a given endometrial cell compartment is thought to be effected by locally released factors. These endometrial polypeptides bind to specific cell surface receptors on target cells, resulting in activation of signal transduction pathways by way of coupling to GTP-binding proteins (G proteins), or through autophosphorylation in response to conformational changes induced by the binding of ligand. Within this paradigm, the highly complex and coordinated expression of decidua-specific genes by differentiating endometrial stroma cells during the late secretory phase of the menstrual cycle could be regarded as the result of the convergence of liganded steroid hormone receptors and specific activated cytoplasmatic signalling pathways.