Deafferented somatosensory thalamic neurons showed hyperactivity, followed by greatly reduced activity, after initiation of cortical spreading depression; local cooling of sensorimotor cortex was followed only by the inactive phase. Stimulation of contralateral midbrain reticular formation during the inactive phase failed to induce the typical increase in discharge rate of somatosensory thalamic neurons, but produced desynchronization in unaffected cortex. These results indicate that corticothalamic discharge is necessary for sustaining the ongoing activity of deafferented somatosensory thalamic neurons and for maintaining their responsiveness to stimulation of the reticular formation.