The medullipin system of BP control has its cellular component in the renomedullary interstitial cells (RICs) of the renal papilla. The RICs secrete medullipin I which is conveyed to the liver to be activated into Med II. This activation appears to involve the cytochrome P-450 dependent enzyme system of the liver. Med II is a vasodilator that suppresses sympathetic tone, causes diuresis-natriuresis and has a suppressive effect on the central nervous system (CNS). As such, Med II has biologic actions the exact opposite of major actions of the renin-angiotensin system. Angiotensin II is a vasoconstrictor that facilitates sympathetic tone, causes salt water retention by the kidney and has stimulatory effect on the CNS. These are double feedback BP control systems. One elevates the BP and is prohypertensive; the other lowers BP and is antihypertensive. Unclipped (UC) the one-kidney, one-clip hypertensive rat has assisted greatly in elucidating the medullipin system. The support for these contentions are documented in this presentation.