1. The rate of calcium transport and calmodulin distribution in the erythrocytes of patients with essential hypertension were studied. In erythrocyte membranes subjected to calmodulin depletion by treatment with EGTA, both the affinity of the calcium pump for Ca 2+ and its maximal activity were the same in normotensive and hypertensive patients. 2. The addition of exogenous calmodulin to calmodulin-stripped membranes from erythrocytes of patients with essential hypertension resulted in a smaller increase of the maximal activity of the calcium pump and its affinity for Ca 2+ . The addition of calmodulin to erythrocyte membranes obtained without EGTA treatment resulted in a smaller increase of the maximal activity of the calcium pump only. 3. There were no significant differences of calmodulin distribution (cytoplasmic concentration and size of the membrane-bound pool) between the erythrocytes of normotensive and hypertensive patients. 4. It is suggested that alterations in the calcium pump activity of the erythrocyte membranes of patients with essential hypertension are related to the alteration of interaction between calmodulin and Mg 2+ , Ca 2+ -ATPase.