Experiments were carried out to determine the role of the sympathetic nervous system in maintaining an elevated blood pressure in chronic renal and deoxycortico sterone/NaCl induced hypertension in the rat. Inhibition of sympathetic nerve function by pithing, mecamylamine and reserpine markedly reduced the blood pressure in both normotensive and hypertensive anaesthetised preparations. When anti-hypertensive agents were given to conscious normotensive and hypertensive rats, only frusemide lowered the blood pressure of the hypertensive rat to the level obtained after similar treatment of the normotensive rat. With the Gillespie and Mum preparation [8] the hypotensive action of some of these drugs was independent of their effects on the peripheral sympathetic nervous system.