The effect of a 5 day ACTH test (40 U/24 h) on plasma aldosterone (aldo), deoxycorticosterone (DOC), plasma renin activity (PRA) and urinary excretion of aldosterone-pH1-conjugate (pH-1-aldo) and tetrahydro-DOC (TH-DOC) was investigated in 8 normotensive children (group I), 8 patients with hypertension of unknown origin (group II), and 4 hypertensive children with dexamethasone suppressible hyperaldosteronism (DSH) (group III). Changes in blood pressure and sodium balance were studied in all groups. Under baseline conditions there was no hormonal difference between group I and II. In contrast, the children in group III had a suppressed PRA and a 1.5--2 fold elevation of aldo and DOC. Plasma DOC and urinary THDOC increased continuously 10--50 fold in all groups during the ACTH test. Aldo rose transiently 2--4 fold on the first day of ACTH and fell subsequently below baseline levels in group I and II. The children with DSH (group III), however, showed an unusual, sustained aldo stimulation with ACTH. PRA decreased significantly after ACTH in group I and II. Sodium retention aTH administration. The highest blood pressure rise was observed in group III (from 124/72 to 139/90 mm Hg). The blood pressure response to ACTH was partly sodium dependent. Although aldo and DOC and sodium retention may contribute to the ACTH induced blood pressure elevation, other factors must play a role.