Voltage clamping giving step commands reveals a steady-state negative resistance characteristic in the current-voltage curves of Aplysia bursting neurons. This is observed below spike threshold in the unstable range through which the membrane potential slowly oscillates. The negative resistance characteristic underlies this instability and shapes the rapid depolarization-hyper-polarization phase of the cycle. When bursting cells are converted to silent cells (by cooling) the negative resistance is abolished; conversely, when normally silent cells are made to burst (by warming) a negative resistance develops. The presence of negative resistance thus enables the bursting cell to oscillate, whereas its absence precluldes such oscillations.