Sodium (Na+) intake increases vascular reactivity. Whether low potassium (K+) intake affects vascular reactivity–associated blood pressure (BP) changes is uncertain. This study aimed to determine whether Na+-induced increases in BP and vascular reactivity are altered by low K+ intake. Male Sprague Dawley rats were assigned to 3 dietary groups for 6 weeks: a standard Na+-K+ diet (control, n = 12), a high Na+-normal K+ diet (HS-NormK, n = 12), and a high Na+-low K+ diet (HS-LowK, n = 12). BP was measured at baseline and after the dietary intervention. Na+ and K+ excretions and vascular reactivity were measured after the dietary intervention. The Na+/K+ ratio was significantly higher in the HS-LowK compared with the other groups. Systolic and diastolic BPs increased significantly in the HS-NormK and HS-LowK groups. In mesenteric arteries, the dose–response curves for phenylephrine-induced contractions shifted to the left and the EC50 (mean ± SD) was significantly lower in the HS-NormK (0.51 ± 0.17 μM, P = 0.003) and HS-LowK (0.69 ± 0.14 μM, P = 0.005) groups compared with the control (3.24 ± 0.79 μM). Systolic (r = −0.58 P = 0.002) and diastolic (r = −0.61 P = 0.001) BPs were associated with the EC50 of phenylephrine-induced contraction in mesenteric arteries. High Na+ intake induces increased alpha-1 receptor responsiveness in mesenteric arteries, which may be responsible for the increase in BP and is not affected by low dietary K+ intake.