Frequency-domain characterizations were determined of the sino-atrial (SA) node heart-rate response to fluctuations in sympathetic and parasympathetic activity in anesthetized dogs. Either the right vagus or cardiac sympathetic nerve was stimulated with a train of supramaximal current pulses whose frequency was modulated by a bandlimited white-noise signal. Transfer functions between nerve stimulation rate and the resulting SA node firing rate were then computed using spectral techniques. It was found that the SA node response characteristics depended significantly on the mean level of vagal or sympathetic tone imposed. The experimental and analysis techniques used are broadly applicable to the investigation of all aspects of autonomic regulation.