Recently, time-frequency analysis has become very popular for examining non-stationary time series and for researching fast changing phenomena. We used the smoothed-pseudo Wigner-Ville distribution to model the underlying dynamic autonomic nervous system changes during the intracarotid sodium amobarbital (ISA) or so-called Wada test. The Wada test involves injecting sodium amobarbitol into the internal carotid artery that results in inactivation of cerebral structures supplied by the ipsilateral anterior and middle cerebral arteries. Electrocardiogram (EKG) data were recorded during the entire procedure and the 10 minutes prior to each injection were used as baseline values for that hemisphere. Interbeat-interval time series were created from these data for a 13 year old patient and were examined using a smoothed-pseudo Wigner-Ville distribution. Inspection of these data indicated that sodium amobarbitol injection to either side produced decreased power in the vagally mediated high frequency band (.14-.40 Hz). Importantly, this decrease was greater when the right hemisphere was inactivated as compared to the left. These results are consistent with the known lateralized innervation of the heart such that right-sided autonomic inputs have greater influence on cardiac chronotropy. The present results also revealed the very rapid changes in autonomic control that characterized the inactivation and subsequent recovery of the cerebral hemispheres. These findings confirm the utility of time-frequency analysis in the investigation of cardiac time series.