A high sodium intake is a risk factor for both hypertension and osteoporosis. High-salt diets cause calciuresis and may influence blood pressure (BP) by stimulating calcium-regulating hormones and augmenting intracellular calcium concentrations. Urinary calcium rises in parallel with urinary sodium. However, aging may influence the rate of excretion of a dietary sodium load. This study compares sodium/creatinine (Na/Cr) values in 2-h urines collected after an overnight fast (12 h) with values obtained in 24-h urines from normotensive women aged (a) 19-23 years (n = 6) and (b) 65-70 years (n = 9) who were consuming either 70 mmol of Na/day [low-sodium regimen (LSR)] or 170 mmol of Na/day [supplemental sodium regimen (SSR)]. Young and elderly women excreted similar amounts of sodium per 24-h period. The 24-h urinary sodium excretion matched dietary sodium input appropriately on LSR and SSR, and SSR raised calcium excretion by 30%. However, whereas in the young women Na/Cr values were similar in fasting and 24-h collections on both LSR and SSR, these values were higher in 24-h than in fasting urines on SSR in the elderly women. We attribute this response to temporal differences in urinary sodium excretion associated with aging. We conclude that assessing sodium excretion from fasting urinary sodium measurements may underestimate total sodium loss (and hence calcium loss) in subjects on a stable sodium intake, particularly in the elderly.(ABSTRACT TRUNCATED AT 250 WORDS)