Plasma atrial natriuretic peptide (ANP) concentrations were measured by both direct radio-immunoassay and with pre-extraction of the peptide from plasma using C18 reverse phase columns. Peptide concentrations were measured in normal subjects (including a group of eight volunteers who received an intravenous infusion of 0.9% NaCl solution), patients with renal failure (including a group with end-stage disease undergoing renal dialysis) and patients with a spectrum of cardiac dysfunction. The overall correlation of results from direct and extracted assay methods was good. However, absolute values from extracted assays were significantly lower than from parallel direct assays. This discrepancy was due to interference from platelets and from another, as yet unidentified, plasma component demonstrated by gel filtration experiments. Extraction of the peptide from plasma by C18 columns largely eliminated these sources of interference and was particularly important for accurate measurement of peptide concentrations within the normal range. Plasma peptide concentrations were elevated in cardiac and renal failure, fell with renal dialysis and rose in normal subjects challenged with an intravenous isotonic fluid load. These findings suggest that ANP participates in the regulation of body fluid volumes and arterial pressure.