Abstract--During early development of hypertension, the spontaneously hypertensive rat (SHR) demonstrates increased proximal tubule sodium reabsorption. Our previous observations of reduced Na⁺,K⁺-ATPase catalytic 1 and subunit transcript abundance in SHR proximal tubule led us to test the hypothesis that increased proximal tubule sodium reabsorption may be attributable to altered subunit protein abundance, post-translational modification, or a shift in subcellular 1 and distribution toward the basolateral membrane. We now extend previous gene expression studies by analyzing total cellular 1 and protein abundance in proximal tubule from SHR compared with matched Wistar-Kyoto (WKY) controls. We also used sucrose density-gradient centrifugation to isolate basolateral, early, and late endosomal membrane-enriched fractions as well as cell surface biotinylation to test the hypothesis of altered subunit subcellular distribution in the SHR proximal tubule. At 4 weeks of age, significantly greater amounts of 1 were present in basolateral membrane-enriched fractions of SHR than WKY (21.1±1.8% versus 12.3±1.8%; P<0.005), and there was a concomitant reduction of 1 in late endosomal membrane-enriched fractions of SHR (63.3±2.7% versus 74.8±4.3%; P<0.05). This finding was confirmed in cell surface biotinylation studies that showed higher 1 (1.45±0.1-fold greater; P<0.05) and -subunit (3.48±0.7-fold greater; P<0.01) abundance in 4-week-old SHR proximal tubule plasma membrane compared with matched WKY samples. These studies support the hypothesis that development of hypertension in SHR may involve an altered subcellular distribution of proximal tubule Na⁺,K⁺-ATPase subunits.