The response of plasma aldosterone (PA) and plasma renin activity (PRA) to ACTH stimulation (0.25 mg Tetracosactide infusion/10 h) and to insulin-induced hypoglycemia (0.1 U/kg b.w.) has been studied in 34 essential hypertensive (EH) patients. Corticotrophin stimulation increases significantly PA, the percent increase being higher in normal PRA EH patients than in controls but comparable to controls in low PRA EH patients. PRA shows a slight and transient elevation. A significant increase in PA is observed also during the insulin test, but the percent increase is lower than that under ACTH stimulation. The possibility that aldosterone is involved, under severe and frequent stress, in the genesis of essential hypertension is discussed.