To date no published data exist regarding the effects of chronic high-dose anabolic-androgenic steroid administration on tonic cardiac autonomic control. The aim of this study was to evaluate, by power spectral analysis of heart rate variability (HRV), the effects of chronic treatment with supraphysiological doses of nandrolone decanoate (DECA) on tonic cardiac autonomic regulation in sedentary rats. Male Wistar rats were treated weekly with 10 mg kg(-1) of DECA (n=7) or vehicle (CONTROL, n=7) for 10 weeks. At the 8th week of treatment, electrocardiogram was recorded in the conscious state, for time- and frequency-domain HRV analysis. Parasympathetic indexes were reduced in DECA group: high-frequency power (CONTROL=11.1+/-3.0 ms2 vs. DECA=3.8+/-0.6 ms2, P<0.05), RMSSD (CONTROL=5.9+/-0.9 ms vs. DECA 3.5+/-0.3 ms; P<0.05) and pNN5 (CONTROL=31.5+/-7.5 ms vs. DECA=13.2+/-2.6 ms; P<0.05). The sympathetic index LF/HF tended to be higher in DECA group (CONTROL=0.65+/-0.15 vs. DECA=1.17+/-0.26, P=0.0546). In conclusion, chronic treatment with DECA, in rats, impairs tonic cardiac autonomic regulation, which may provide a key mechanism for anabolic steroid-induced arrhythmia and sudden cardiac death.