1. In order to examine the potential role of endogenous atrial natriuretic peptide (ANP) in modulating the increased sodium excretion per nephron in chronic renal failure, we studied healthy subjects with normal renal function (group I) and patients with moderate (group II) or severe chronic renal failure (group III) before, during and after administration of an intravenous sodium load. All subjects had been on a controlled diet containing 120 mmol of sodium per day for 5 days before the study. 2. Under basal conditions, plasma ANP and fractional excretion of sodium (FENa) were highest in group III. Both parameters increased in response to the sodium load in the three groups studied (P less than 0.001). Changes with time differed from group to group (P less than 0.05), the more marked response for both parameters being observed in group III. After adjustment with respect to plasma ANP (analysis of covariance), FENa was no longer modified in response to the sodium load, whereas adjustment of FENa with respect to mean blood pressure was without consequence on the significance of its change with time. This demonstrates that plasma ANP, but not mean blood pressure, represents the main factor producing variation in FENa during and after the sodium load. 3. These results suggest an important role for plasma ANP in promoting adaptation of short-term sodium excretion in response to an acute sodium load in patients with chronic renal failure who ingest a normal sodium intake.