To determine the effects of isotonic volume overload on cardiac autonomic function and hemodynamic parameters in patients scheduled for spinal anesthesia which benefi ciated of pre-anesthetic hydration. Iin all patients, initially, before starting the pre-anesthetic volume loading, there where a state of cardiac hyper-sympathicotonia. First 10-15 minutes after starting fl uid administration, both sympathetic and parasympathetic cardiac tonuses increase. At the end of hydration, after all liquid volume was administered (10-15 ml/kg), vegetative sympathetic cardiac tonus was signifi catively reduced, meanwhile parasympathetic cardiac tonus was reduced non-signifi catively, being higher than that registered before pre-anesthetic hydration. Hemodynamic indices (TAs, TAd, MAP) increase progressively during volume expansion, meanwhile the pulse reduces progressively with hydration.Isotonic volume loading in pre-anesthesia is associated with reduced spectral component of LF, this refl ecting decreased sympathetic modulation on the heart and increased spectral component HF – refl ecting increased parasympathetic (vagal) modulation on the heart. This can be explained by arterial-cardiac barorefl ex coupling, which is manifested hemodinamically by progressive increase of TAs, TAd, MAP, and progressive reduction of heart rate.