The effect of artrial natriuretic polypeptides on the intracellular free. calcium concentration of immune cells of spontaneously hypertensive rats has been investigated. It was found that artrial natriuretic polypeptides could raise directly the intracellular free calcium concentration of thymocytes, splenocytes and lymphonode cell, or cooperate with Con A in increasing the intracellular free calcium concentration of rat lymphocytes and T lymphocytes except for B cells. It was also found that artrial natriuretic polypeptides could cooperate with IL-2 in increasing the intracellular free calcium concentration of rat lymphoblast induced by Con A. The extent of increased intracellular free calcium concentration induced by artrial natriuretic polypeptides in immune cells of spontaneously hypertensive rats was lower than that of WKA rats.