High amounts of calcium in the urine (hypercalciuria) can cause development of kidney stones in children. Treatment for these children includes plenty of fluids, a low-salt diet and medications such as potassium citrate. A major advantage of potassium citrate, as compared to hydrochlorothiazide, is its lack of side effects. One problem the researchers and others have observed is that some children continue to form kidney stones despite correction of hypercalciuria with potassium citrate. One possible explanation is that in some individuals potassium citrate therapy results in an excessive elevation of urine pH, a situation that may predispose to calcium phosphate stone formation. In this study, the researchers will study the effects of potassium citrate on urine chemistries and acid-base balance in three groups of children aged 5-17 years: