Fibromyalgia is a clinical condition with several manifestations that compromise autonomic modulation in patients who suffer from this disease, and therefore affects heart rate variability. Sympathetic hyper-activation and/or autonomic dysfunction have been described in fibromyalgia patients. PURPOSE: The main purpose of the study was to evaluate the heart rate variability responses in fibromyalgia patients compared to healthy controls. METHODS: A short registration of heart rate variability responses was performed during four stages: rest, controlled ventilation at 10 and 12 cycles per minute (CV10, CV12), dynamic exercise (30 squats in 45 seconds) and recovery post activity. The subjects were all female, seven patients with fibromyalgia diagnosed by a rheumatologist, according to standardized criteria, and seven healthy controls matched by age. None of the subjects were taking any medications that could compromise the autonomic response (n=14). The age range was between 26 and 66 years old. All test were made between 7 and 10 am. The high frequency (HF), and low frequency (LF) domains of the spectral analysis were compared between groups, together with the LH/HF ratio and the root of mean square differences between adjacent R-R intervals (RMSSD), using a t student test. RESULTS: There were important changes in the autonomic response reactivity in the control group but not in the fibromyalgia patients. The variables that show significant differences were the HF domain at the CR10 (p = 0,0042) and CR12 (p=0,0268), the LF/HF ratio during the supine position (p=0,0296) and the heart rate, also at the supine position (p=0,0042). CONCLUSIONS: These results show a diminished autonomic reactivity during different stages of the test, in fibromyalgia patients but not in the control group, which indicates not only an increased basal sympathetic activity, but also parasympathetic dysfunction modulation. These findings clarify the need to make and individualized and different approach with fibromyalgia patients, specially prescribing physical activity.