The retinal inputs to cells in the cat's lateral geniculate nucleus (LGN) were directly recorded to study the basis for the properties of two classes of LGN X-cells: Xs (single) and XL (lagged). The presence of excitatory or inhibitory input to an LGN cell from a particular simultaneously recorded ganglion cell was assessed with cross-correlograms during unstimulated activity. Because neighboring ganglion cells do not fire independently, features in a retinogeniculate correlogram can arise in two ways that must be distinguished by a direct effect of the ganglion cell on the LGN cell, or by correlated firing between that ganglion cell and some other ganglion cell that is an excitatory or inhibitory input to the LGN cell. It was possible to determine the origin of correlogram features because features indicating a retinogeniculate effect were distinctly different in timing and strength from features arising solely from correlated firing in the retina. The characteristic feature in a correlogram between an LGN cell and an excitatory retinal input was a sharp peak in LGN cell firing rate at the appropriate latency after the firing of the ganglion cell. The characteristic feature for an inhibitory input was a dip in LGN cell firing rate after the firing of the ganglion cell. Typically, this dip lasted 10-40 ms and was followed by a prolonged enhancement in LGN cell firing rate, which may reflect a postinhibitory rebound. XS-cells had a single retinal X input whose excitatory effect caused most of the LGN cell's spikes during stimulated and unstimulated activity. There was no conclusive evidence that any XS-cell received excitatory retinal input from either Y-cells or other X-cells of the same center sign. There was usually evidence for inhibition of XS-cells by retinal X-cells of opposite center sign with receptive fields highly overlapping that of the XS-cell, but rarely evidence for inhibition by Y-cells. XL-cells also had only a single excitatory input, but this X input had a relatively weak effect that caused only a minority of the LGN cell's spikes, typically 17% during maintained activity and 29% during visual stimulation. The input's excitatory effect was immediately followed by strong inhibition of the XL-cell. XL-cells were also inhibited by retinal X-cells of the same center sign that were adjacent (nearest neighbors) to the excitatory input. The strength and latency of both of these inhibitory effects indicate that the inhibition was disynaptic.(ABSTRACT TRUNCATED AT 400 WORDS)