Attempts will be made to produce an experimental model of essential hypertension in the rat using combinations of behavioral and physiologic means. The fluid intake produced by beta-adrenergic agents will be investigated in terms of its possible non-dependence upon the renin-angiotensin system and mechanism of action mediated by the renal medulla. Characterization of the drinking induced by diazoxide and phosphodiesterase inhibitors will be explored using various agents including the angiotensin II analog P-113 and converting-enzyme inhibitor SQ 20881. The limits of diazoxide-induced dilutional hyponatremia resulting from repeated diazoxide administration and the ensuing polydipsia will be measured and characterized by appropriate serum and urine determinations.