The effect of short- and long-term sodium loading and sodium restriction on the gene expression as well as on circulating plasma levels of atrial natriuretic factor (ANF) was evaluated in normotensive Wistar rats. These rats were fed either a low-, a regular-, or a high-sodium diet (regular diet and 1% saline as drinking fluid) and studied after 1 and 3 wk. The ANF mRNA was determined in pooled atria and ventricles of the different groups of rats, using the dot-blot technique. Plasma ANF levels were measured with a radioimmunoassay. After 1 wk on the high-sodium diet, ANF mRNA was increased in right atria and ventricles together with circulating ANF levels when compared with animals maintained for the same period on a low-sodium diet. After 3 wk on the various diets, the differences in cardiac ANF mRNA and in plasma ANF levels had disappeared. Gene expression of ANF was also looked for in different areas of the brain, lung, thyroid, adrenals, and the kidney; no hybridization was detected in any of these organs. These data suggest that in rats, the transcription of the ANF gene and peptide release in enhanced only during short-term adaptation to dietary sodium loading.