Objective: To test the following three hypotheses: 1 plasma sodium concentration displays a 24-hour rhythm; 2 the circadian rhythm of plasma sodium is disrupted in SHR fed a basal NaCl diet; 3 high NaCl diet elevates plasma sodium concentration and may lead to an increase in mean arterial pressure (MAP) of SHR. Methods: After having been instrumented with arterial catheter at 9 weeks of age, all rats (Spontaneously Hypertensive Rat, SHR; Wistar-Kyoto Rat, WKY) experienced two experimental protocols, the mean arterial pressure and heart rate were monitored and the blood samples also been analysed for sodium contents. Results: On basal NaCl diet, plasma sodium concentrations in SHR were significantly higher than those of WKY at almost every time point measured and the plasma sodium concentration and MAP rhythms were in nearly opposite phases; on high NaCl diet, the MAP and plasma sodium rhythms were essentially opposite in WKY. In SHR, the plasma sodium rhythm was inversely related to the MAP rhythm. Conclusion: Plasma sodium displays a significant circadian rhythm in rats. The high NaCl diet disrupts the normal plasma sodium circadian rhythm only in SHR.