The endomembrane Ca(2+)-ATPase inhibitor, thapsigargin, was used to deplete the intracellular Ca2+ stores of fura-2-loaded human platelets. In control cells, thapsigargin evoked a rise in cytosolic [Ca2+] and a substantial increase in protein tyrosine phosphorylation. Thapsigargin also evoked an increase in tyrosine phosphorylation in cells co-loaded with fura-2 and the Ca2+ chelator dimethyl BAPTA, such that the rise in cytosolic [Ca2+] was abolished. These data support the existence of a tyrosine phosphatase regulated by the Ca2+ content of the intracellular store, a requirement of the putative model for reciprocal control of Ca2+ entry by cytosolic and store [Ca2+] via protein tyrosine phosphorylation.