The production of persistent changes in the level of neuronal activity by brief local cooling of the cerebral cortex of the rat.

1. Local cooling of the surface of the cerebral cortex of the rat gives rise to an increase in the spontaneous firing rate of cortical neurones.2. When this increase in firing is maintained, by continued cooling, for 5 min or more, a prolonged after-effect consisting of an increase in rate above the control level is produced lasting up to at least 3(1/2) hr following restoration of temperature.3. The potential gradient across the cortical surface during cooling is in the opposite direction to that required to produce an increase in firing during and after trans-cortical polarization.4. It is therefore likely that the important factor in producing after-effects is associated with the actual firing rate of the neurones during the cooling (or polarizing) procedure rather than the accompanying extracellular potential gradient.