The biochemistry of the renin-angiotensin system has been outlined. The relationship between plasma angiotension II levels, basal or raised as a result of infusion, and blood pressure and aldosterone has been explored in normal human subjects and animals, showing how these relationships can be modified by changes in sodium status and duration of exposure to high circulating levels of the octapeptide. Finally, the relationship has been examined in different categories of hypertensive disease, such as essential hypertension.