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In 1966 Burke and Sefton1 3 reported that interneurons (Icells) of the rat lateral geniculate body (LGB) were identifiable as the units showing repetitive discharges to single shock stimulation of either optic nerve or visual cortex. This response pattern of I-cells was clearly distinguishable from that of principal cells (P-cells) which were fired mostly once by optic nerve stimulation and antidromically by visual cortical stimulation. Several pieces of evidence, though indirect, led these workers to assume that I-cells might be inhibitory neurons acting upon P-cells and intercalated between the latter and their axon collaterals. Although several workers noted4>-6> that Burke and Sefton's I-cells were recordable when unit discharges were searched for in and around LGB, no works have been published on their location. In the present experiment a systematic exploration for I-cells was made and it was found that this type of cells were clustered in a zone separate from but immediately neighboring to the one containing P-cells exclusively. This anatomical arrangement of P-and I-cells made it Dossible to examine effects of electrical stimulation of the cluster of I-cells upon individual P-cells. The results were that activity of P-cells, either spontaneous or evoked by optic nerve stimulation, suffered suppression upon activation of I-cells. Methods. Rats, weighing 200-300 g, were used. After initial anesthesia with an intraperitoneal injection of urethane (1 g/kg), the rats were fixed to a stereotaxic apparatus by Fifkova and Marsala's method.7~ Subsequently a small dose of urethane was given intraperitoneally as frequently as required. One per cent procaine was injected to all pressure points. A bipolar stimulating electrode, consisting of singly aligned insulated wires with exposed tips separated by 0.5 mm, was introduced each into the optic tract (OT) at the optic chiasm and into the optic radiation (OR) about 2 mm below the visual cortex. Stimulus pulses *' Visiting scientist from Electrotechnical Laboratories ,

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