Increasing evidence suggests that the renin-angiotensin system (RAS) is not only a potent regulator of blood pressure and fluid and electrolyte homeostasis, but that it also plays an important role in growth and differentiation in development as well as in pathological states. We, therefore, investigated the expression of all components of the RAS in the human embryo and fetus by in situ hybridization or immunohistochemistry. This study is the first to demonstrate the presence of all components of the RAS in very early human development (30-35 days of gestation). Angiotensinogen mRNA is expressed in very high amounts in the yolk sac, liver, and kidney, whereas renin mRNA and angiotensin-converting enzyme are expressed in the chorion, kidney, and heart, thus allowing fetal production of angiotensin II. This effector molecule of the RAS mediates its effects through binding to specific receptor types, AT1 and AT2. Both of these receptors are also expressed very early in development (24 days of gestation), suggesting a role for angiotensin II in organogenesis. Based on the expression pattern of these receptors, angiotensin II likely plays a role in the growth and differentiation of the kidney, adrenal gland, heart, and liver, all organs that are of major importance for the regulation of blood pressure later in life.