Although it is generally believed that the beneficial effect of loop diuretics is the result of a rapid increase in diuresis, substantial evidence, from a large number of in vitro and in vivo experiments, has accumulated showing that administration of furosemide causes direct vascular effects, which probably contribute to its acute clinical effects. Several mechanisms are involved in the vascular response to loop diuretics. The role of the renin-angiotensin-adolsterone axis, prostaglandins and the direct vascular effects of loop diuretics on both the arterial and venous parts of the vasculature are discussed.