Possible influences of posture or age on plasma immunoreactive atrial natriuretic peptide (irANP) levels and potential correlates were assessed in 12 young (age ± s.e.m. 24 ± 1 year) and 12 elderly (63 ± 8 year) healthy subjects on a liberal sodium intake. The groups did not differ significantly in their basal 24-h urinary sodium excretion (210 ± 23 versus 180 ± 15 mmol/l). However, plasma irANP was five- to ninefold higher in the elderly (P P P < 0.01) to 250 ± 51 and 50 + 9 pg/ml, respectively, after intravenous (i.v.) loading with 0.9% saline (2.14 I in 3 h). Supine blood pressure (BP) and plasma norepinephrine tended to be higher while renin and aldosterone levels were lower (P < 0.01) in the elderly; the three latter variables rose (P <0.001) with upright posture. These findings demonstrate that in normal humans, circulating irANP levels vary with posture and ageing. These changes may have potential physiological relevance and should be considered when interpreting plasma irANP levels in pathological conditions