Analysis of heart rate variability is a valuable method to investigate the sympathetic and parasympathetic function of the autonomic nervous system. Although such analyses can provide quantitative estimates of autonomic neural activity, simultaneous recording of neural activities and ECG will allow more direct investigation of neural modulation of heart rate. We developed a method that allows direct and long-term recording of neural activities and ECG using wireless device implanted in ambulatory dogs. The aim of this study was to investigate the relationship between autonomic neural firing and heart rate variability. In this study, HRV and neural activities were assessed for 5 continuous days. HRV was evaluated by calculating the mean and the standard deviation of inter-beat intervals in 24 hours. Neural activities were obtained by the sum of the filtered rectified neural signals after 200 Hz high-pass filtering to remove ECG interference. The plots showing HRV as a function of both the sympathetic and vagal activities will offer significant insights into neural modulation of heart rate in normal and diseased hearts.