In this little essay I describe recent advances in understanding the problem of salt sensitivity and salt resistance involved in the control of blood volume and blood pressure. Genetic evidence links the recently characterized epithelial sodium channel (ENaC) and the potassium channel (ROMK-1) to monogenic diseases in humans, characterized by a renal salt-losing syndrome. A loss of function mutations in ROMK-1 gene causes in some pedigrees the syndrome of Bartter, characterized by metabolic alkalosis and a severe salt-losing syndrome. A loss of function mutations in ENaC genes causes pseudohypoaldosteronism-type 1, characterized by hypovolaemia, hyperkaliaemia, metabolic acidosis and hypotension. ENaC and ROMK-1 are expressed in the apical membrane of principal cells of the cortical collecting duct and their role in Na/K balance is briefly reviewed.