Abstract: Although the avian retina has long been known to receive projection from a midbrainnucleus, the isthmo-optic nucleus (ION), the output of its target cells has remained obscure.We labeled the isthmo-optic (IO) terminals in the Japanese quail retina, by using anterogradetransport of fluorescent tracer injected into the ION, and then labeled target cells for theseterminals by means of intracellular tracer injection under direct microscopic observation.Somata of the IO target cells (IOTCs) lie in the innermost zone of the inner nuclear layer ofthe ventral half of the retina and have no dendrites but an axon. The axons run in the innerplexiform layer (IPL) for up to 6 mm and terminate densely in a round or elliptical terminalfield, about 90-290 um in diameter, of the outermost zone of the IPL. Longer axons (>2 mm)extend dorsally, but shorter ones (<1 mm) project ventrally or horizontally, so the terminalsare distributed widely in both dorsal and ventral halves of the retina. The IOTCs cannot beclassified into any of the five conventional major classes of retinal cells, including amacrinecells, and are thought to be "slave" neurons whose output is controlled by the neurons in thebrain. Topographic separation between input to and output from the IOTCs by the axonsmight be essential for the overall topographic organization of the centrifugal visual system in birds.