Background/Aims: A recent observation that antiplatelet-aggregation drugs, including ticlopidine hydrochloride, may prevent erythropoietin (EPO)-induced rise in blood pressure in hemodialysis (HD) patients remains a subject of particular interest. The aim of the present study was to determine the effect of ticlopidine hydrochloride on EPO-induced rise in blood pressure of HD patients with special reference to blood levels of vasoactive substances. Methods: HD patients who showed hypertension or aggravation of preceding hypertension with EPO treatment were selected for this study. Ticlopidine hydrochloride was administered at a dose of 200 mg daily for 4 weeks. Blood pressure and serum levels of nitric oxide (NO), atrial natriuretic peptide (ANP) and endothelin (ET) were determined before and after drug administration. Patients were divided into two groups, one of which showed a drop in mean blood pressure (MBP) of >10 mm Hg (group I) and one which did not (group II), and a comparison was made between them with respect to the blood parameters. Results: Five of 15 patients showed a drop of MBP of >10 mm Hg (group I), and 10 patients did not show any change in MBP (group II). In group I, there was a significant increase in blood NO levels compared to the concentrations before ticlopidine administration, while there was no change in group II. With respect to ANP and ET, there was no significant change in either of the groups. Conclusion: The findings suggest that the preventive effect of ticlopidine hydrochloride on EPO-induced rise in blood pressure may partly be related to the enhancement of NO production in patients on maintenance HD.