This study was designed to determine whether expression of renal messenger RNA (mRNA) encoding the two known angiotensin II type 1 (AT₁ ) receptor subtypes (AT_1A and AT_1B) can be regulated by dietary sodium. Seven-week-old male Wistar rats were fed a low-sodium diet (0.07%, n=9) or a normal-sodium diet (0.5%, n=9 [control]) for 14 days. A rat AT₁ complementary DNA (cDNA) probe, which hybridizes to mRNA encoding both the AT_1A and AT_1B receptor subtypes, and cDNA probes, which are selective for AT_1A or AT_1B mRNA, were used in Northern blot or in situ hybridization analysis. By use of Northern blot analysis, renal mRNA levels for the AT₁ and AT_1A receptors in rats fed a low-sodium diet were found to be increased twofold (P<.05) compared with control. Because renal AT_1B mRNA content was not detected by Northern blot analysis, quantitative image analysis of in situ hybridization with a digoxigenin-labeled cRNA probe made from AT_1B cDNA was used. In situ hybridization analysis indicated that AT_1B mRNA was expressed in the proximal and collecting tubules of the kidney in rats fed a normal-sodium diet. The low-sodium diet significantly decreased the percent positive staining area of AT_1B mRNA in the renal cortex (5.51±0.77% versus 2.73±0.35%, P<.05) and medulla (4.76±0.70% versus 2.01±0.43%, P<.05) compared with the control diet. These results indicate that the increase in AT₁ mRNA levels in the kidney induced by low sodium intake is the result of a selective increase in AT_1A mRNA and suggest that AT_1A is the predominant receptor subtype of AT₁ in the kidney. The data also suggest that dietary sodium differentially modulates the expression of genes encoding AT₁ receptor subtypes, because there is an inverse relationship between the expression of the AT_1A and AT_1B subtypes in response to a low-sodium diet. The functional implications are discussed.