The aim of the study was to evaluate the adaptation of the circulatory system based on the HRV at rest and at steady state. They studied 33 women (34–36 years old), who exercised on the steady-state level of 70–80% of maximum heart rate. ECG was recorded using a Polar RS800 at rest and during effort. Analyzed parameters: RMSSD, pNN50%, HF%, VLF%, LF%, LF/HF. All test results compared to the rest of effort showed statistically significant differences. The following conclusions was stated: HRV may be used to evaluate the adaptation of the circulatory system; during steady-state exercise exists reducing the impact of parts of parasympathetic and increasing the influence of sympathetic part the autonomic nervous system; analysis of HRV indicates the increasing influence of the renin–angiotensin system and thermoregulation during exercise.