['Atrial natriuretic peptide (ANP) is a hormonal regulator of cardiovascular fluid volume. More than 1,000 scientific articles were written about ANP between 1987 and 1991. Because some articles hinted at problems with storing ANP, this study examined the effect of numerous techniques for storing and processing human ANP samples.', 'Samples were obtained repeatedly from three patients, treated, and stored under a variety of conditions. Experiment 1 evaluated the effects of different preservatives at 35, 21, 14, 10, and 7 days before assay. Experiment 2 evaluated nonspecific binding of ANP to different storage tubes during 28 days of storage. Experiment 3 evaluated the effect of storage at -20 degrees C, -80 degrees C, and -196 degrees C for 1 month. ANP was very unstable, degrading as much as 30% after 3 days of storage and by more than 50% in 1 month even when stored at -80 degrees C. Only storage at -196 degrees C (in liquid nitrogen) kept ANP stable for 1 month. Extraction and lyophilization of the samples before freezing and assay within 7 days of freezing only partially minimized the amount of degradation. All other processing techniques had little effect on slowing the degradation of ANP.'] Is plasma atrial natriuretic peptide unstable under most storage conditions?