A high expression of leukotrienes receptors (CysLT2) has been detected in Purkinje fibers of the heart conduction system and in cardiomyocytes. The hereby research is the first attempt to evaluate the action of leukotrienes (LTs) on the heart conduction system. We investigated the effect of pharmacological inhibition of 5-lipooxygenase (the enzyme of leukotriene synthesis pathway) on LTs production and the cardiac electrical activity in patients with stable coronary heart disease subject to elective coronary angiography with possible percutaneous angioplasty and in patients with permanent atrial fibrillation. In this trial zileuton (5-lipoxygenase inhibitor) was applied and ECG Holter recording was performed with consecutive measurements of urine LTE4 concentrations. It has been shown that in patients with stable coronary heart disease who have coronary angiography or coronary angiography with angioplasty with stent implantation performed, there is a transient activation of the 5-lipooxygenase pathway and increased synthesis of cysteinyl lukotrienes. Pharmacological inhibition of leukotriene biosynthesis in patients with stable coronary heart disease subject to intracoronary interventions or patients with permanent atrial fibrillation, causes: reduction of the mean 24-hour heart rate, increase in parameters of heart rhythm variability (in patients with sinus rhythm), and an improvement in the conduction of electrical impulses in the conduction system during intracoronary procedures. Inhibition of 5-lipooxygenase does not affect the incidence of arrhythmia, nor the degree of heart ischaemia.