The biologic actions of the cardiac peptide hormone atrial natriuretic peptide (ANP) of vasorelaxation, diuresis and natriuresis, suppression of aldosterone, vasopressin release, and thirst are the opposite of those of the renin angiotensin system. This close relationship is further strengthened by the complementary localization of their receptors in the brain, adrenal gland, vasculature, and kidney. In many physiologic situations including postural changes, volume expansion, water immersion, high altitude, and lower body negative pressure, the plasma levels of ANP and angiotensin II change inversely. In congestive heart failure, renin and aldosterone levels may initially be suppressed by high levels of ANP. Similarly the low renin levels associated with increasing age and with elderly hypertensive patients, may be the result of the elevation of plasma ANP that occurs with aging. ANP may thus be the endogenous antagonist of the renin angiotensin aldosterone system. These two opposing systems allow fine-tuning of volume and pressure by the body.