This article describes the time domain and spectral analysis of heart rate variability (HRV) to characterize the response of the autonomic nervous system (ANS) in renal patients when dialyzing fluid at 35 degrees C and 37 degrees C is used during hemodialysis (HD). The study group was 10 patients in which 3 ECG recordings were acquired, one at the beginning, after 1 hour and at the end of HD, obtaining the corresponding RR series to each measurement. Subsequently time domain indices: mean, SDANN, HRVindex and Load(index), and the spectral index LF / HF were calculated and analyzed statistically using the Friedman test to determine which temperature causes more significant changes. The results show that there were significant differences occurred in the control of ANS in HD patients when dialyzing liquid is used at 35 degrees C or 37 degrees C being the SDANN the most sensitive index to temperature change.