SUMMARY OF WORK. This research investigates the role of breathing pattern and dietary sodium intake in blood pressure regulation and the development of chronic hypertension. It is based on previous work which found that high sodium intake elevated blood pressure in genetically-normotensive large animals, but only if they were under behavioral stress that generated intermittent suppression of breathing, increases in pCO2, transient decreases in plasma pH, renal sodium retention, and increases in urinary marinobufagenin excretion, a natriuretic factor that inhibits sodium pump activity and increases vascular tone. Subsequently, blood pressure of human subjects with high resting end tidal CO2 was found to be more sensitive to high sodium diet than that of persons with low resting end tidal CO2. Women who reported more chronic stress were found to breathe more slowly at rest than those who reported less chronic stress. This association was less marked in men. The positive association of high resting end tidal CO2 with resting blood pressure was found to be greater in older women than in older men. We reported during the past year that increases in systolic blood pressure and pulse pressure of 16 normotensive postmenopausal women on seven days of high sodium diet following low sodium intake was positively correlated with increases in asymmetric dimethylarginine, an endogenous inhibitor of nitric oxide, that plays an important role in natriuresis. Individual blood pressure response to salt was also inversely correlated with changes in endothelium-dependent vasodilation, as measured during reactive hyperemia to nitroglycerine. In addition, we reported that transdermal estrogen attenuated the blood pressure response to high sodium diet in postmenopausal women. The findings indicate that estrogen might improve endothelial function by reducing oxidative stress and nitric oxide breakdown. An experimental study that tests the hypothesis that breathing pattern is a risk factor for sodium sensitivity of blood pressure in normotensive women is in progress. This study can clarify the mediating role that behavioral stress might play in the association of high sodium diet with blood pressure, and lead to rational behavioral interventions for the prevention of some forms of human hypertension.