Evidence for the existence of a hormone that is stimulable by adrenocorticotropic hormone (ACTH) and capable of causing hypertension has been collected in several patients. This hormone is not a known mineralocorticoid or glucocorticoid. The hypothesis that a steroid can produce hypertension was tested in an 18-year-old man with dexamethasone-suppressible hypertension. During dexamethasone treatment, when aldosterone secretion was suppressed, less than normal and the patient was normotensive, steroids were given by constant infusion in an attempt to reproduce the hypertension of the dexamethasone-free state. Hypertension was not caused by 5 days of administration of aldosterone, 18-hydroxydeoxycorticosterone (18-OH-DOC) at 1 mg/day, or deoxycorticosterone (DOC) at 30 mg/day. However, sodium retention and potassium loss were observed during infusion of aldosterone and DOC. Hypertension was produced within 5 days during infusion of ACTH or oral metyrapone. The hypertensive effect of the metyrapone was eliminated by the additional treatment with aminoglutethimide. These studies suggest that an ACTH-dependent steroid rather than aldosterone, 18-OH-DOC, or DOC may be the cause of the hypertension in this patient. Study of a 3-year-old child who presented with short stature, hypertension, hypokalemic alkalosis, suppressed renin and ACTH, and decreased excretion of all known steroids suggested excessive secretion of a pressor hormone. Reversal of the hypertension and hypokalemic alkalosis occurred when spironolactone was administered. ACTH exacerbated the clinical and biochemical abnormalities, suggesting that the secretion of the unknown factor was dependent on ACTH. A study of the urinary steroids revealed remarkably low excretion of aldosterone and cortisol. Plasma levels of ACTH were low. The low production of aldosterone was not associated with the increased excretion of precursor metabolites. These finding suggest the secretion of an unknown hypertensive factor of remarkably high potency, with the ability to suppress the secretion of both renin and ACTH.