1. Thromboembolism occurs in 0.4-2.0% of patients undergoing radiofrequency catheter ablation (RFCA). Some studies have shown that treatment with heparin inhibits the activation of coagulation and fibrinolysis. No study has directly measured the activation of platelet aggregation to investigate the effect of heparin on platelet function. The purpose of the present study was to observe the inhibitory effect of heparin on platelet activity in patients undergoing RFCA. 2. Sixty-two patients with supraventricular tachycardia were observed and divided into a heparin-treatment group and a control group. Changes in platelet aggregability (PAG) and thromboxane B(2) (TXB(2)) in the blood samples of all patients at different times (before, after electrophysiological study, immediately after and 10 and 30 min after the RFCA procedure) were observed. 3. No indication of clinically symptomatic thromboembolism and no major differences in baseline characteristics and procedure were apparent in either group. The levels of PAG and TXB(2) were all clearly increased after the electrophysiological study (all P < 0.05). Immediately after RFCA, PAG and TXB(2) levels were significantly increased in both groups and remained elevated 30 min after the procedure (all P < 0.05). In the heparin-treatment group, the increases in PAG (54.69 +/- 3.24%) and TXB(2) (29.01 +/- 1.84%) caused by RFCA were lower than changes observed in the control group (70.92 +/- 3.45 and 44.70 +/- 3.28%, respectively; both P < 0.01). Moreover, treatment with heparin normalized the elevated level of PAG 30 min after RFCA more clearly. 4. The results of the present study suggest that intravenous heparin treatment during the operation inhibits the activation of platelets induced by RFCA.