1. The sodium and potassium conductances of the HODGKIN-HUXLEY model are simulated by a field effect transistor with a series resistor. This arrangement leads to a simple analog model of the excitable membrane (fig. 1 and 2). 2. Normally, the model is silent (fig. 3), but it becomes automatic (fig. 4) when the decay time (de-activation) of the potassium conductance is at least twice the recovery from inactivation time of the sodium conductance (taud greater than 2 tauri). 3. The effects of changes in sodium (fig. 5 and 6) and potassium (fig. 7, 8 and 9) concentration gradients upon the membrane potential and the ionic currents are easily studied when the model is silent or automatic. 4. When automatic, an increase in the potassium concentration gradient induces a lengthening of the period and ultimately, when the gradient is very high, spontaneous activity is blocked (fig. 9). On the other hand, increases of sodium gradient over 30% of normal value do not modify the period (fig 6). 5. The potassium concentration gradient modifies the excitability solely through membrane polarization (fig. 8), while sodium concentration has no effect on it (fig. 5). 6. Results with the model strengthen the hypothesis that tetraethylammonium (TEA) acts on both the maximum potassium conductance (gK) and the mechanism of sodium conductance inactivation (Tauh) to lengthen the action potential as observed on the Ranvier node (fig. 10). Effects of TEA on potassium conductance activation are also discussed. 7. Because of its simplicity and accuracy, this model lends itself easily to many other simulations.