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Blood sodium concentration [Na] was changed over a limited range of about ±15mmol/l by intraperitoneal dialysis with physiological salt solutions containing variable amounts of Na. The transmembrane distribution of Na, K and water in vivo was measured in rapidly excised tail arteries. Cell Na varied linearly by about 0.3 mmol/kg dry weight for a 1 mmol/l change in extracellular sodium concentration ([Na]0). Expressed as the concentration in cell water ([Na]j), the change was about 0.4 mmol/l for a 1 mmol/l change in [Na]0. The Na gradient varied inversely with [Najj. Systolic and diastolic blood pressures were directly related to [Na], and inversely related to the Na gradient. We conclude that this relationship is part of a causal chain regulating blood pressure. The effect of a change in Na partition was blunted by the concurrent change in osmolarity which affects vascular smooth muscle tension by a different mechanism acting in the opposite way. We suggest that this latter mechanism is based on the special geometry of the cell, which elongates when water is withdrawn and shortens when water is imbibed.