The effect of a continuous 5-day ACTH infusion (40 U/24 h) on adrenocorticoid function, electrolyte metabolism, and blood pressure was investigated in eight normotensive children and eight patients with hypertension of unknown origin. There was a continuous rise of plasma cortisol and deoxycorticosterone in all patients. Plasma aldosterone rose transiently in the normotensive and the hypertensive group. A transient kaliuresis and a continuous fall in serum K+ were observed in all patients. ACTH induced sodium retention and weight gain. The observed increase in systolic blood pressure correlated significantly with the cumulative sodium retention in the normotensive and the hypertensive groups. No correlation between sodium retention and diastolic pressure was found. ACTH on a low salt diet (10 meq/24 h) produced a blood pressure rise which was smaller than that on regular salt. The blood pressure rise did not correlate with any of the hormones measured. This study provides evidence for an unidentified ACTH-stimulable adrenal factor capable of raising blood pressure in normotensive children and patients with juvenile hypertension. The ACTH-induced blood pressure rise is only partly salt dependent and the mechanism of the rise remains unclear.