Ketanserin, an antagonist of peripheral serotonin receptors when given to hemodialyzed patients treated with recombinant human erythropoietin (rHuEpo) corrects some changes in hemostasis but also apparently delays an increase in hematocrit. We wished to elucidate the effects of oral administration of ketanserin on serum Epo, some hematological and biochemical blood parameters, arterial blood pressure (BP), and bleeding time in hemodialyzed patients receiving rHuEpo therapy. We noted a 33% decrease in Epo concentration (p < 0.05) after a 4-week ketanserin trial in patients in the initial stage of rHuEpo therapy. Although a concomitant decrease in erythrocyte count and hemoglobin did not reach statistical significance, these changes correlated positively with decreasing Epo level (r = 0.749 and 0.787, respectively). Ketanserin administered for 14 days to patients between 32 and 34 weeks of rHuEpo therapy also produced a decrease of 26% in Epo concentration (p < 0.005). This decrease correlated (r = 0.629) with a decrement in the red blood cell (RBC) count (p < 0.005). Hemoglobin concentration followed the same pattern (p < 0.005). However, the decreases in the reticulocyte count did not reach statistical significance. The decrease in hormone concentration resulted in a concomitant thrombocyte decrease (p < 0.05) only in patients who received ketanserin in the interval between 8 and 12 weeks of rHuEpo therapy. The previously normal bleeding time was significantly prolonged (p < 0.05) in both groups of patients. There were no changes in leukocyte count, iron status parameters, or calcium, phosphorus, or bilirubin concentration. Administration of ketanserin even for 4 weeks did not influence BP in the patients.(ABSTRACT TRUNCATED AT 250 WORDS)