1. To investigate whether the difference in the SA gene expression in the kidneys is causally related to the pathogenesis of hypertension, we reassessed the expression of the SA gene in the kidneys of the spontaneously hypertensive rat (SHR), its stroke-prone substrain (SHRSP) and Wistar-Kyoto (WKY) rat from different sources (SHR/Izm, SHRSP/Izm and WKY/ Izm from Izumo colony; SHR/Crj and WKY/Crj from Charles River Laboratories). 2. At the age of 5 weeks, high levels of the SA mRNA were expressed in the kidneys of SHRSP/Izm, SHR/Izm, SHR/Crj and WKY/Izm, while very low levels of the SA mRNA were observed in those of WKY/Crj. At the age of 8 weeks, the expression of the SA mRNA in the kidneys of WKY/Izm was at the same level as in those of SHRSP/Izm and two SHR strains. 3. Four genetic markers at the SA locus, an StuI restriction fragment length polymorphism and three microsatellite markers, were not polymorphic among Izumo strains of SHR, SHRSP and WKY rats. 4. In situ hybridization showed strong signals of the SA mRNA in the renal proximal tubules, while no positive signals were detected in the glomeruli. 5. Because WKY/Izm has normal blood pressure, our observations indicate that a simple difference of the SA gene expression in the kidney cannot be an explanation for the difference of blood pressure between SHR(SP)/Izm and WKY/Izm.