In the cat striate cortex, single electrical shocks applied to the dorsocaudal claustrum (CLdc) elicited bimodal excitatory responses with about 12 and 26 ms latencies. About one-fourth of the cortical cells observed had CLdc-induced inhibitions with onset latencies longer than the excitations. On cortical field responses to geniculate stimulation, CLdc conditioning shocks exerted early facilitatory and late inhibitory effects which were shown to be not transmitted through the mesencephalic reticular formation.