A large body of evidence has accumulated which demonstrates that sodium transport in some distal nephron segment is altered in response to changes in extracellular fluid volume. Sodium reabsorption in the loop of Henle and distal tubule is directly related to delivery rate and is not inhibited by volume expansion. In contrast, recent studies have shown that Ringer loading causes a greater natriuretic response than hyperoncotic albumin because of diminished collecting duct sodium transport in the former model. Additional studies in animals with different basal extracellular fluid volumes and in DOCA-escape rats indicate further that the collecting duct is an important regulator of sodium balance. Although the factors that modulate sodium transport in the collecting duct are not clear, it is postulated that the local release of prostaglandins may be of major importance.