BACKGROUND: Parkinson\'s disease (PD) is associated with autonomic dysfunction and chronic levodopa therapy has been reported to impair the autonomic control of heart rate. AIM: Our aim was to assess the immediate effect of a single dose of levodopa on heart rate variability (HRV) in idiopathic PD. MATERIALS AND METHODS: Eleven patients of idiopathic PD (F:M =2:9, mean age 57.3+/-8.6 years, duration of illness 4.1+/-2.8 years, Hoehn and Yahr stage 2.1+/-0.2) on stable levodopa dosage were studied. Motor part of unified Parkinson\'s disease rating scale and resting Lead II electrocardiogram (ECG) recordings were performed at baseline (12 hours off medication) and after two tablets of 100/10 mg of standard levodopa/ carbidopa. ECG was recorded continuously in the first hour (H1) followed by a 15-min recording in second (H2), third (H3) and fourth (H4) hours. Artifact free 5-min segments of the ECG were analyzed offline to obtain the HRV parameters in time domain (ms) and frequency domains (ms 2 ). RESULTS: Significant increase was observed in standard deviation of normal to normal intervals (23.5+/-2.7-46.2+/-6.6, P<0.05), root mean square of successive differences of NN intervals (16.3+/-2.9-30.7+/-5.1, P<0.01), total power (568.9+/-125.7-2739+/-667.5, P<0.01), low frequency power (146.5+/-40.8-614.1+/-206.7, P<0.05) and high frequency power (107.4+/-33.9-332.7+/-85.9, P<0.05) in H1. CONCLUSION: The results are suggestive of an improvement in the overall variability of the heart rate indicating an enhanced vagal tone.