The present study aims to answer the following questions: 1. do secretion of volume related hormones in patients with EH pre- and post treatment with propranolol differ from normotensive subjects if examined in thermal dehydration conditions; 2. is the electrolyte composition of thermal sweat related to the plasma profile of volume related hormones? and 3. does treatment by propranolol influence sweat electrolytes in EH patients. In 15 patients with EH and in 20 healthy subjects a thermal dehydration test was performed. In patients with EH this test was done twice: before treatment and after 6 weeks of propranolol therapy. In all subjects the plasma renin activity (PRA), aldosterone (Ald), AVP and ANP were measured before and after thermal dehydration. In sweat samples collected after 15' and 45' of thermal dehydration (the concentration of Na, K and Cl was assessed). In hypertensive patients before propranolol treatment significantly higher values of PRA, Ald and ANP were found, while sweat concentrations of Na and Cl were significantly lower than in controls. After propranolol treatment sweat electrolytes concentrations showed a tendency to normalize. No significant correlation was found between the plasma hormonal profile and sweat Na, K and Cl concentrations respectively both in controls and patients with EH pretreatment. A significant positive correlation was noticed only in hypertensive patients posttreatment between ANP and sweat potassium concentration respectively, and significant negative correlation between PRA and sweat sodium and chloride concentration. From results obtained in this paper it seemed, that volume related hormones (Ald, AVP, ANP) do not seem to influence markedly the electrolyte composition of thermal sweat both in healthy subjects and in hypertensive patients.