An electronic model of a cardiac pacemaker cell is developed from a study of the Hodgkin—.Huxley equations describing the dynamic behaviour of excitable tissue membranes. The model is based on the functional hypothesis that the controlling factor is the leakage conductance of the membrane to sodium ions and that this is increased by stimulation of the sympathetic nerves and decreased by stimulation of the parasympathetic nerves. The resulting model shows close agreement with experimental results obtained by Levy and Ziesko on the heart of the anaesthetised dog. The model forms part of an circulatory-parameter analogue of the human circulatory system currently being constructed as an aid to studying the manner in which the parameters and control loops of this system adapt to prolonged periods of heavy exercise.