The aim of this study was to investigate time-course of cardiac autonomic activity in rats treated with nandrolonedecanoate (DECA). Twenty male Wistar rats received weekly 10 mg.Kg-1 of DECA (DG) or vehicle (CG) during 8 weeks. To heart rate variability analysis SDNN (standard deviation of RR intervals), RMSSD (square root of the mean squared differences of successive RR intervals), pNN5 (percentage of successive RR interval differences greater than 5ms) and Poincaré analysis was applied in time-domain. Additionally, power spectral was decomposed into high (HF: 0.8-2.5Hz) and low-frequency (LF: 0.2-0.85Hz). For the HF component, differences were found between DG and CG during 4th, 6th and 8th weeks. Progressive increase in the LF-HF ratio was found in DG, resulting in significant differences between groups in 4th, 6th, 7th and 8th weeks. DG showed decreased from 24.5±7.2ms2 to 3.9±2.55ms2 in HF corresponding (83.9% of alteration). Power spectral density function for HF corresponded to 26.3±8.3% and 26.5±4.1% in the 1st and 8th weeks (P=NS) for CG, whereas the DG showed reduced significantly from 26.4±3.6% (1st) to 18.99±2.7% (8th week). Cardiac autonomic dysfunction may constitute an early consequence of DECA administration and an important marker of arrhythmia vulnerability and sudden death identification.