Successful human pregnancy is associated with striking physiological changes. Plasma volume increases early in the first trimester, attaining maximal expansion near term, when the increment above non-pregnant values is about 1.5 L (50–60%), and is a contributory factor in maintaining organ perfusion in the presence of arteriolar and venous dilatation. Effective renal plasma flow (ERPF) and glomerular filtration rate (GFR) increase to values 50–70% above non-pregnant levels by term. There are also gestational alterations in osmoregulation as well as in the renal handling of sodium, potassium and glucose, coincident with altered tubular function. Plasma volume expansion is significantly related to fetal growth and pregnancies complicated by intrauterine growth retardation and pre-eclampsia are associated with reduced plasma volume and impaired renal function. Elucidation of factors involved in volume homeostasis and renal function in pregnancy is therefore of clinical as well as physiological importance. Atrial natriuretic peptide (ANP) causes vasodilatation, natriuresis and inhibition of the renin-angiotensin system and is clearly an important factor in volume homeostasis and renal function.