Investigations were carried out on the behavior of 18-hydroxy-11-deoxycorticosterone (18-OH-DOC) in essential hypertension (EH) under exogenous administration of synthetic ACTH and insulin. 40 stable EH patients and 21 normal subjects were included in the study. The increase (12-fold basal values) in plasma 18-OH-DOC in normal subjects under Tetracosactide was significantly higher than cortisol (4-fold basal values). Furthermore, insulin hypoglycemia increased 18-OH-DOC levels 5-fold, whilst basal values of cortisol were increased 2-fold. An increase in 18-OH-DOC and cortisol was also observed in EH patients: in the subgroups with normal and low plasma renin activity, however, the rise in these two steroids was significantly lower than that in normal subjects both under Tetracosactide and insulin. No significant hormonal modifications were observed after furosemide administration either in the normal subjects or in the EH patients. 18-OH-DOC by itself does not, therefore, appear to play a pathogenetic role in EH.