The mammalian lateral geniculate nucleus seems organized to gate or control the gain of retino-geniculate transmission, the result of which is then relayed to the visual cortex. We have performed in vivo intracellular studies of retinogeniculate transmission along these retino-geniculo-cortical pathways in cats by recording the retinally evoked excitatory postsynaptic potential (EPSP) in geniculate neurons. In cats, these pathways are organized into two parallel and functionally distinct channels, the X and Y pathways. We found that nearly all geniculate X cells display a fairly conventional voltage dependency for their retinally evoked EPSPs, because the amplitudes of these EPSPs decrease fairly linearly with membrane depolarization as the EPSP reversal potential is approached. Rare X cells and all Y cells, however, show an unconventional response: over a wide range of membrane potentials, their EPSP amplitudes increase with membrane depolarization. This increase does not result from alterations in neuronal input resistance and instead seems due to changes in synaptic conductance. The underlying cause of this voltage dependency remains to be determined. None the less, it does afford an interesting means by which retinogeniculate transmission can be gated, since non-retinal inputs (e.g. corticogeniculate axons) that can control a relay Y cell's membrane potential can also modulate the cell's EPSP amplitude.