Sodium balance was studied in 7 and 16 week old male spontaneously hypertensive rats (SHR), in matched normotensive Wistar rats (NCR) and in Wistar Kyoto rats (WKR). The animals were placed in metabolic cages and given diets with either normal sodium content (5.35 mmol sodium/100 g food) or with a sodium content 3 or 10 times the normal. Whether on normal or increased sodium diet we did not observe any increased sodium retention in either SHR age group. However, in both SHR groups urinary sodium excretion was correspondingly increased compared with the controls. This shift of sodium excretion from kidneys to gastrointestinal tract in SHR did not reflect any 'primary' inability of the SHR kidneys to excrete sufficient sodium amounts since on high sodium diet they excreted the increased sodium load as readily as the normotensive controls. The present results do not support the concept that a primary renal retention of sodium and water should be of pathogenetic importance for the SHR variant of primary hypertension.