The role of the renin-aldosterone system in the etiology of hypertension due to coarctation of the aorta was evaluated in pre-operative adolescents hospitalized in the Pediatric Clinical Research Center. Five patients aged 12-20 years were studied. Blood pressure was 146/82 to 170/115 in 4 patients and 130/84 mm Hg in a 12 year old. Variable sodium (Na) diets were administered during hospitalization. Patients received normal Na (100-150 meq/day), low Na (10 meq/day), and high Na (250 meq/day) diets for a minimum of 3 days/diet. Potassium (K) intake was maintained at 50 meq/day. Daily 24-hour urinary aldosterone (aldo), Na, and K excretions were obtained. Plasma renin activity (PRA) was measured at rest and after ambulation. Patients received no medication. All were in K balance and had normal serum K. The aldo response to changes in dietary Na was qualitatively and quantitatively normal when compared to the nomogram of New et al. The response of PRA to differential Na diets was also qualitatively normal. However, PRA was more variable than previously reported in adults. In some patients, the response of PRA to ambulation and/or low Na diet was higher than reported adult standards. The significance of this response requires further evaluation. We conclude that the renin-aldosterone system was intact in these hypertensive adolescents with coarctation of the aorta.