SummaryMineralocorticoid therapy was discontinued in 14 subjects, ages 7–17 years, with salt-losing congenital adrenal hyperplasia. The level of plasma renin activity (PRA) was determined in relation to the cumulative loss of sodium (intake-urinary output) over 4–12 days. Although the magnitude and rate of response varied, PRA increased in all subjects. There was a significant positive correlation between cumulative sodium loss and PRA (r= 0.585,P< 0.001,n= 76), and a significant negative correlation between serum sodium concentration and PRA (r= - 0.736,P< 0.001,n= 76). Significant sodium loss and elevation of PRA were seen prior to the development of hyponatremia. Thus, elevated PRA appeared to be a more sensitive indicator of sodium loss than serum sodium concentration.Fifteen infants, ages 1–33 months with salt-losing congenital adrenal hyperplasia, who were treated with subcutaneous pellets of 11-deoxycorticosterone acetate (DOCA), were also studied. PRA decreased following implantation of the DOCA pellets over 1–6 months. In most instances there was a subsequent increase in PRA, beginning at 1–8 months post implantation. There was a significant negative correlation between serum sodium and PRA (r= −0.669,P< 0.001,n= 78) in these infants. However, there were four instances in which elevated PRA was associated with normal serum sodium.Sodium balance studies were also performed on 14 of these infants during 29 admissions. All balance studies were performed at approximately 3 or 6 months after DOCA pellet implantation. On a diet containing 2 mEq Na/kg body weight per day, sodium balance was positive and there was no significant change in sodium excretion over the 3 days of observation. When a 1 mEq Na/kg/day diet was given, sodium balance was neutral on day 3 of the study, and fractional urinary sodium excretion (FeNa) decreased from 0.49 ± 0.33, S.D. on day 1 to 0.23 ± .09% on day 3 (P< 0.001,n= 19). This change was statistically significant. Net sodium balance on either diet was independent of PRA. Elevated PRA was not necessarily associated with adverse symptomatology.Thus in infants treated with DOCA pellets, elevated PRA was associated with indices of sodium depletion and decreased availability of DOCA. The dietary sodium requirement for infants treated with DOCA appeared to be 1–2 mEq/kg body weight/day.