Hyperpolarization of the ventrolateral thalamic cell membrane reveals a slow inward current which is not normally observed at the resting membrane potential. The response evoked by depolarizing synaptic potentials or depolarizing current pulses from a hyperpolarized potential consists of a burst of action potentials superimposed upon a slow voltage response, in contrast to the single active response evoked without the background polarization. We propose that such behavior is caused by a slow inward current that is activated at subthreshold potentials and inactivated or masked at resting potential.