<p>This study aims to clarify the process of Na⁺ overaccumulation under saline-alkaline (SA) stress at the&#xa0;initial stage of the stress periods. Koshihikari rice plants at the four- to five-leaf stage were subjected to saline stress (50&#xa0;mM Na⁺; pH 5.5) and SA stress at various pH levels (50&#xa0;mM Na⁺; pH 7.5 (mild), 8.0 (moderate), and 8.5 (severe)), then the concentrations of Na⁺ and K⁺ and gene expressions in the samples were measured. The Na⁺ concentrations in the roots, young-leaf-sheaths (YLS), young-leaf-blades (YLB), and old-leaf-sheaths (OLS) under mild, moderate, and severe SA stress were increased more sharply within 24&#xa0;h of stress treatments than under saline stress. In the old-leaf-blades (OLB), the Na⁺ concentrations increased rapidly under saline and SA stress at lower pH compared to that under SA stress at higher pH. The plasma membrane Na⁺/H⁺ antiporter <i>OsSOS1</i> (for Na⁺ exclusion and Na⁺ loading from xylem) was highly expressed in the roots and YLS under saline and mild SA conditions, while it was downregulated in the OLS under saline, mild SA, and moderate SA conditions. The vacuolar Na⁺/H⁺ antiporter <i>OsNHX1</i> (for vacuolar Na⁺ sequestration) tended to be induced by SA stress in the roots. High pH promoted the drastic accumulation of Na⁺ in the roots and young leaves at the initial stage of the stress period under salinity conditions, possibly due to variations in expressions of genes related to Na⁺ transport in response to saline and SA stresses. These findings are crucial for understanding the characteristics of Na⁺ accumulation patterns in rice under SA stress.</p>