Morphological and cytochemical features of peripheral blood thrombocytes from patients with paroxysmal tachyarrhythmias refractory to treatment with antiarrhythmic drugs were studied using modern techniques of fluorescence and electron microscopy. Blood samples of 33 patients were analyzed, 21 of them being refractory to treatment. Blood samples of 5 healthy persons served as controls. Nonactivated, activated, degranulated thrombocytes and their aggregates were counted in blood smears. Concentration of noncompensated negative charges in glycocalyx and activity of adenylate cyclase, phosphodiesterase of cyclic nucleotides and monoamine oxidase were determined by ultrastructural histochemistry techniques and mean content of alpha-granules was determined morphometrically. The morphofunctional state of thrombocytes in patients with refractory tachyarrhythmias was found to change significantly resulting in an increase in the number of irreversibly activated and degranulated forms and glycocalyx modification. These changes are supposed to represent an important component of the pathogenesis of resistance to drug treatment. The level of patients' sensitivity to the drug therapy correlates with the parameters under study indicative of changes in the thrombocyte surface and ultrastructure and of the number of free-circulating thrombocyte complexes. Under these conditions the deposition function of thrombocytes in restricted. The reactivity of platelets is also changed due to appearance of amorphous deposits on their surfaces which either protect or block the receptors of the cells.