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To test the hypothesis that blood pressure of older adults with high resting end tidal CO2 (PETCO2) is sensitive to high dietary intake of sodium chloride.and methods Forty-four Caucasian men and women, aged 41-79 years (mean ± SEM 55.5 ± 1.4), restricted their dietary intake of sodium chloride for 11 days and ingested sodium chloride capsules (an additional 190 mmol sodium/day) during the last seven of those days. On days 1, 4 and 11, resting PETCO2 and blood pressure were monitored for 25 min in the laboratory, followed by ambulatory blood pressure monitoring in the natural environment for 24 h. Overnight urine samples were obtained at days 4 and 11 to estimate excretion of sodium and of an endogenous digitalis-like factor (EDLF) that is sensitive to changes in plasma volume.Individual resting PETCO2 remained stable within and between laboratory monitoring sessions and was correlated with urinary excretion of the endogenous digitalislike factor, both before and after the period of high sodium intake. The high-sodium diet was associated with increased urinary sodium excretion and body weight in all quartilesof PETCO2. The high-sodium diet produced significant increases in resting and in 24 h systolic blood pressures in the upper two quartiles of the PETCO2 distribution, and significant increases in resting and in 24 h diastolic blood pressures in the highest PETCO2 quartile only.These findings indicate that blood pressure sensitivity to sodium loading is differentially associated with high resting PETCO2 in older adults. A high PETCO2 may be an index of a dynamic steady state that influences sodium transport mechanisms.