In this chapter are outlined the many factors involved in the regulation of sodium and volume homeostasis in normal human pregnancy and their interrelationships. New developments concerning the role of sodium/potassium ATPase, atrial natriuretic peptide, arginine vasopressin and angiotensin II as regulatory forces are outlined, together with a review of earlier work. Abnormalities found in women with, or destined for, PAH are described and their significance is discussed.