To test the hypothesis that abnormal platelet Ca²⁺ handling in essential hypertension results from cellular Mg²⁺ deficiency, cytosolic free Mg²⁺ concentration ([Mg²⁺]_i) and Ca²⁺ metabolism were studied in mag-fura 2 and fura 2-loaded platelets from 30 essential hypertensive patients and 30 sex- and age-matched normotensive controls. Basal cytosolic free Ca²⁺ concentration ([Ca²⁺]_i) and intracellular Ca²⁺ discharge capacity were higher in hypertensives than in normotensives (22 ± 5 vs. 18 ± 5 nM, P < 0.05; 743 ± 250 vs. 624 ± 144 nM, P < 0.05, respectively). The thrombin (0.03-1.0 U/ml)-evoked [Ca²⁺]_i response was also enhanced in platelets from hypertensives in both the absence and presence of extracellular Ca²⁺. However, basal [Mg²⁺]_i was higher in hypertensives than in normotensives (437 ± 110 vs. 353 ± 85 µM, P < 0.05), whereas serum Mg²⁺ was similar in the two groups. These results oppose the Mg²⁺ deficiency hypothesis in platelets in essential hypertension.