Stimulation in the segment of nucleus reticularis thalami adjacent to the lateral geniculate body (RLG), abolished visual evoked potentials for up to 150 msec. Both photic stimulation in the contralateral visual field and electric stimulation in the ipsilateral optic tract elicited primary cortical responses that were markedly reduced or abolished by prior conditioning stimulation in RLG. Stimulation of the segments of nucleus reticularis thalami adjacent to the medial geniculate (RMG) or the ventrobasal complex (RVBC) had the effect of markedly reducing or abolishing unilaterally projected primary evoked responses in the auditory and cutaneous systems, respectively. Only the sensory evoked potentials mediated by the relay nucleus adjacent to the region of R stimulated were affected. The reduction of the cortical evoked potentials was not due to the processes underlying the cortical recovery cycle, because conditioning stimulation on either side of RLG stimulated the primary geniculocortical fibers, but had a minimal or no effect on the visual test evoked response. These results suggest that R functions as a topographically organized inhibitory gate which can regulate the patterns of sensory input from the thalamus to the cortex. The regulatory effects on R by the mesencephalic reticular formation and the mediothalamic-frontocortical system may mediate both generalized and selective control of cortical sensory evoked potentials.