Altered activity of sympathetic nervous system is one of the potential factors influencing blood pressure elevation during erythropoietin (rHu-EPO) therapy in patients with end-stage renal disease (ESRD). The aim of study was to establish if rHu-EPO administration to ESRD patients affects heart rate variability (h.r.v.) indices in the time domain and activity of the autonomic nervous system (a.n.s.). 23 ESRD hemodialyzed patients were divided into those who recived rHU-EPO (+EPO), N = 12 and did not receive rHu-EPO (-EPO), N = 11. +EPO patients were given 2000-4000 IU rHu-EPO/week for 6 weeks. In both groups h.r.v. indices (mRR, SDNN, rMSSD and pNN50) were calculated during dialysis sessions and interdialytic period and then were analyzed by Cosinor method. It was shown that: 1) during dialysis session there were rhythmic changes in all h.r.v. parameters, which can be described by Cosinor formula. 2) all h.r.v. indices wee lower in +EPO group as compared with -EPO patients. 3) the differences, although not significant, may indicate sympathetic activation in +EPO ESRD patients. 4) Cosinor method applied to h.r.v. analysis may be useful tool for assessing a.n.s. activity.