Magnesium (Mg) is a vasodilator which may play a role in the regulation of blood pressure. The present study measured tissue Mg and calcium (Ca) levels in a hypertension model, the spontaneously hypertensive (SHR) rat, using the Wistar-Kyoto rat as a control. Mean serum Mg levels were normal in both types of rats. In SHR rats given H2O ad libitum, four of nine tissues (kidney, heart, lung, bone) had significant 6-16% decreases in Mg content (less than 0.025-0.0005). In SHR rats undergoing chronic saline diuresis, seven of nine tissues (liver, kidney, testis, heart, lung, spleen, bone) had significant 7-18% decreases in Mg content. Tissue Ca in these same rats was significantly decreased by 6-39% in four of nine tissues, but was increased in other tissues. These observations indicate that both Mg and Ca depletion can occur in selected tissues in SHR rats and the number of tissues with Mg depletion doubles during chronic saline diuresis. If Mg depletion is also found in the vascular smooth muscle of this hypertension model, it may be a contributory factor in the hypertension.