Evidence has accumulated for the existence of local tissue renin-angiotensin systems in the reproductive tissues. In the ovary the local renin-angiotensin system (RAS) has been shown to influence ovulation and steroidogenesis. In this review we focus on aspects of a local RAS in the uteroplacental unit. High renin concentrations have been found in all tissues of the uteroplacental unit. The presence of renin mRNA in the endometrium, choriodecidua and the fetal part of the placenta indicates local renin synthesis. Angiotensin (Ang) II, formed by the enzymatic action of tissue renin and Ang I converting enzyme, acts by interaction with its receptors. These receptors have been demonstrated in high densities in the placenta and uterus, indicating an autocrine or paracrine action of Ang II. Several probable effects of the uteroplacental RAS can be defined. It is very likely that the uteroplacental RAS plays an important role during implantation and placentation by stimulation of decidualization and angiogenesis. Furthermore, Ang II may regulate synthesis and secretion of other hormones formed locally in the uteroplacental unit. During labour the action of Ang II may be important for contraction of the uterine musculature, Ang II is also involved in the complex regulation of the uteroplacental blood flow. Species difference in the expression of the uteroplacental RAS exists, and one may speculate that the physiological and pathophysiological roles vary between species.