Atrial natriuretic peptide (ANP) levels were studied in young (2-week-old) and old (age 20 months) rats following acute (20 mL/kg bolus of normal saline i.v.) vs. chronic sodium loading (high-salt diet for 21 days). In both groups serum ANP rose significantly 1 h post i.v. loading. However, the increment was greater in the old animals. On chronic sodium load, in the young group, ANP rose significantly on days 3 and 7, subsequently declining to baseline. In the old animals, no increase in ANP was noted throughout the chronic experiment. Twenty-four-hour urinary sodium excretion was similar in the two groups following acute or chronic salt load. The results suggest that: (1) The old kidney requires a greater ANP stimulus to excrete a given acute i.v. sodium load. (2) The relatively diminished elasticity of the old heart plays a major role in the exaggerated ANP release in response to a bolus i.v. load of saline. (3) The role of ANP in maintaining homeostatic renal sodium excretion during short-term sodium oral loading is different in the two age groups.