Water-free Na⁺ storage may buffer extracellular volume and mean arterial pressure (MAP) in spite of Na⁺ retention. We studied the relationship among internal Na⁺, K⁺, water balance, and MAP in Sprague-Dawley rats, with or without deoxycorticosterone-acetate (DOCA) salt, with or without ovariectomy (OVX). The rats were fed a low-salt (0.1% NaCl) or high-salt (8% NaCl) diet for 5 weeks. DOCA salt increased MAP (161±14 versus 123±4 mm Hg; P<0.05), and DOCA-OVX salt increased MAP further (181±22 mm Hg; P<0.05). DOCA salt increased the total body Na⁺ by &40% to 45%; however, water-free Na⁺ retention by osmotically inactive Na⁺ storage and by osmotically neutral Na⁺/K⁺ exchange allowed the rats to maintain the extracellular volume close to normal. DOCA-OVX salt rats showed similar Na⁺ retention. However, their osmotically inactive Na⁺ storage capacity was greatly reduced and only partially compensated by neutral Na⁺/K⁺ exchange, resulting in greater volume retention despite similar Na⁺ retention. For every 1% wet weight total body water gain, MAP increased by 2.3±0.2 mm Hg in DOCA salt rats and 2.5±0.3 mm Hg in DOCA-OVX salt rats. Because water-free Na⁺ retention buffered total body water content by 8% to 11% wet weight, we conclude that this internal Na⁺ escape buffered MAP. Extrarenal Na⁺ and volume balance seem to play an important role in long-term volume and MAP control.