We previously demonstrated that tonic stimulation of either the sympathetic or the vagal nervous system augmented the dynamic heart rate response to the other of the two systems. We characterized the phenomenon as bidirectional augmentation of heart rate regulation. The question remained unanswered, however, as to whether such augmentation could occur under simultaneous dynamic stimulation of the two systems. The transfer characteristics from nerve stimulation to heart rate were well described by linear systems analysis, although no attention was paid to the aphasic nature of the stimuli in relation to each R-R interval. When we stimulated the two nerves with statistically independent Gaussian white noises, gain of the transfer function increased by 63.2 +/- 47.4% relative to individual stimulation (P < 0.05). When we stimulated the two nerves with mutually reciprocal Gaussian white noises, gain of the transfer function increased by 54.9 +/- 49.1% (P < 0.05). Thus simultaneous dynamic stimulation of the sympathetic and vagal systems bidirectionally augmented heart rate regulation irrespective of the pattern of the stimulation signals.