The expression of the intrarenal renin angiotensin mRNAs in Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) were investigated in rats fed a low (0.02%)- or normal (1.6%)-sodium diet for 5 days. Total RNA was isolated from the kidneys and analyzed by both Northern and slot blots. The results indicated that the kidneys of all four groups of rats containing readibly detectable levels of renin and angiotensinogen mRNAs. The kidneys of the WKY contained higher levels of angiotensinogen mRNA under normal-salt diet compared with the SHR (P less than 0.01). Mild sodium depletion stimulated angiotensinogen mRNA in WKY kidneys by almost 50% compared with normal salt diet (P less than 0.01). In contrast, there was no significant difference in the renal angiotensinogen mRNA levels in the kidneys of SHR fed a low- or normal-sodium diet. Renin mRNA concentrations were comparable in both strains. Mild sodium depletion failed to yield any detectable changes in renin mRNA levels in either strain. Thus the SHR exhibits an alteration in the sodium regulation of intrarenal angiotensinogen mRNA expression. These results may have implications in the renal physiology of these animals.