Using a microprocessor controlled Ambulatory Monitoring System (AMS) developed by one of us (LvD), we have been studying the changes in and control of heart rate in the resting horse. The system provides us with InterBeat Intervals (IBI in milliseconds), motion sensing, and a time domain measure (mean successive differences: MSD) of heart rate variability for periods up to 72 hours. Thoracic impedance is also available but parameters for the equine chest are not currently available. The system is completely noninvasive, small, and carried on a surcingle worn by the subject. The equine subject is confined to a stall in our teaching hospital but not otherwise restrained. Recording is virtually unobtrusive. Ten horses (judged to be clinically normal) were used in this preliminary study. After collection, the data were "offloaded" to a laptop computer for additional analysis. The electrocardiogram could be recorded on each of the ten animals. Complete data, suitable for spectral analysis, were obtained from four of the animals. Spectral estimates were calculated on periods of varying lengths (3-5 minutes) with more stable spectral estimates associated with longer recording periods. Results indicated the preponderance of parasympathetic control in equid heart rate. These results provide support for the utility of this method for the study of heart rate variability in the freely behaving horse.