In normal human pregnancy there is considerable expansion of both plasma volume and total extracellular fluid volume, together with marked diminution of vascular reactivity in response to intravenous infusion of vasopressor agents. Pregnancy-associated hypertension (pre-eclampsia) is characterised by increased vascular reactivity, vasoconstriction and a fall in plasma volume associated with a shift of fluid from intravascular to interstitial fluid space. Direct expansion of plasma volume in patients with pregnancy-associated hypertension leads to partial correction of the abnormalities of volume homeostasis together with a significant fall in blood pressure. Certain forms of antihypertensive therapy also lead to reversal of volume contraction and improvement in cardiac function. This is associated with lowered vascular resistance and improved placental function, leading to improved pregnancy outcome. Although these observations do not elucidate the ultimate cause/causes of pregnancy-associated hypertension, they help to unravel the pathologic mechanisms by which the syndrome causes its effects and to suggest rational paths by which therapeutic endeavours may be expected to benefit mother and fetus.