The aim of the study was to investigate chosen haemostasis activation markers during electrophysiologic study (EPS) with consequent radiofrequency catheter ablation (RFA). Sixty-three patients were studied prospectively. Indications for EPS and RFA were supraventricular tachycardias with the arrhythmogenic substrate located in the right atrium. Blood samples were drawn 24 hours before the procedure (T -1), at the beginning of the procedure (T0), at the end of EPS (T1), 30 minutes after completion of RFA (T2), and 24 hours after the procedure (T3). To study coagulation, fibrinolytic and platelet activation were measured concentrations of thrombin-antithrombin III (TAT), D-dimers (DD), platelet count and parameters, and circulating platelet aggregates (CPAi). During the EPS and RFA, TAT levels increased from the baseline 5.03 +/- 2.53 microg/l (T -1) to 12.90 +/- 12.83 microg/l at T0 (p < 0.001) to 36.07 +/- 15.59 microg/l at T1 (p < 0.001) and decreased to 28.85 +/- 13.14 microg/l at T2 (p < 0.001). Levels of DD increased from 0.30 +/- 0.20 mg/l at T0 to 0.44 +/- 0.25 mg/l at T1 (p < 0.001) and to 0.87 +/- 0.74 mg/l at T2 (p < 0.001). The number of platelets was significantly decreased (-13.7%) before and during the procedure (T -1 vs. T3; p < 0.001). Marked platelet activation (CPAi 0.62 +/- 0.32) was observed before the procedure opposite to the physiological values (CPAi 1.0 +/- 0.1), without changes during the procedure (CPAi at T2 0.69 +/- 0.23). Our results confirmed activation of several haemostasis parameters during EPS and RFA, and support eligibility of the antithrombotic prevention in patients indicated for EPS and RFA.