Freezing of gait (FOG) is one of the most disabling symptoms that affect patients with Parkinson's disease (PD). While the patho-physiology underlying FOG largely remains an enigma, several lines of evidence suggest that the autonomic nervous system might be involved. To this end, we tested the hypothesis that heart rate (HR) increases during FOG and, further, that HR increases just prior to FOG. To evaluate these hypotheses, fifteen healthy older adults, ten patients with PD who experienced FOG and ten patients who did not were studied. Patients with PD were tested during their “off” medication state. HR and HR variability were measured as subjects carried out tasks that frequently provoke FOG. 120 FOG episodes were evaluated. During FOG, HR increased (p=0.001), by an average of 1.8 bpm, as compared to HR measured before the beginning of FOG. HR also increased just prior to FOG, by 1 bpm (p<0.0001). In contrast, during sudden stops and 180° turns, HR decreased by almost 2 bpm (p<0.0001). HR variability was not associated with FOG. To our knowledge, these findings are the first to document the association of FOG to autonomic system activation, as manifested by HR dynamics. One explanation is that the changes in HR before and during FOG may be a sympathetic response, secondary to limbic activation, that contributes to the development of freezing. While further studies are needed to evaluate these associations, the present results provide experimental evidence linking impaired motor blockades to autonomic nervous system function among patients with PD.