It is well known that epoetin alfa. increases serum endothelin (ET)-1 and blood pressure. No data are available, however, on the effects of darbepoetin alfa on serum ET-1 and blood pressure. This study was conducted to compare the effects of darbepoetin alfa and epoetin alfa on serum ET-1 and blood pressure in patients on hemodialysis (HD). A total of 42 patients on HD were included in the study. Serum samples for measuring levels of ET-1 were taken 30 min after administration of epoetin alfa. After blood samples had been taken from all patients, epoetin alfa was changed to darbepoetin alfa. Three months after the start of darbepoetin alfa treatment, blood samples were taken to measure the same parameters. Mean arterial blood pressure was measured before recombinant human erythropoietin (EPO) administration and 30 min after EPO administration while patients were taking epoetin alfa or darbepoetin alfa. Injection of epoetin alfa or clarbepoetin alfa significantly increased serum ET-1 levels compared with levels in those patients who were not on EPO therapy (P <.05). When the effects of epoetin alfa on serum ET-1 level were compared with those of darbepoetin alfa, the 2 types of EPO were found to increase serum ET-1 levels similarly (P >.05). Administration of epoetin alfa or clarbepoetin alfa increased systolic and diastolic blood pressures significantly over values in the control group (P <.05). Serum systolic and diastolic blood pressures increased similarly after injection of epoetin alfa or darbepoetin alfa. Administration of clarbepoetin alfa increased blood pressure in patients on HD in a way that was positively correlated with enhanced ET-1 release; a similar correlation was noted with epoetin alfa.