The plasma levels of nine vasoactive regulatory peptides were measured by radioimmunoassay in six stable patients with chronic renal failure on regular hemodialysis, before and during treatment with recombinant human erythropoietin (r-huEPO). All patients responded with significant increases in hemoglobin concentrations and hematocrit. Mean arterial blood pressure was not significantly changed nor were there any changes of body weight or interdialytic body weight gain. The mean plasma levels of atrial natriuretic peptide and motilin decreased significantly, by 38 and 16 percent respectively, during r-huEPO treatment. There were no changes in mean plasma levels of arginine vasopressin, calcitonin gene-related peptide, beta-lipotropin, gamma 2-melanocyte-stimulating hormone, neuropeptide Y, substance P or vasoactive intestinal peptide. No significant correlations were observed between changes of plasma peptide levels and changes of mean arterial blood pressure.