The first section of the model delineates the parasympathetic signal as a function of the interaction between the respiratory and the vagal neurones of the common brainstem network. The second section is a version of Chess and Calaresu's description of the vagus/heart period system in the sympathetically blocked cat. The sympathetic influence is introduced in the third section, taking into account the autonomic effects on the transmembrane potential of the sinoatrial pacemaker as found in the rabbit. The model was implemented on a dedicated analogue computer with an accuracy of 1 per cent. The concordance between the analogue and the breathing/respiratory sinus arrhythmia (RSA) system in humans was studied using simulated respiratory signals. The results confirm the central origin of the RSA as a meaningful modelling premise and indicate the dependence of the RSA amplitude on the operating point of the vagus.