Relay cells in laminae A and A1, of the cat lateral geniculate nucleus (LGN) can be classified as X- or Y-cells. In addition, a group of LGN relay cells appearing to show properties intermediate to those of X- and Y-cells (IM-cells) has been reported. This paper describes the receptive-field organization of X-, IM-, and Y-cells and the results of a multivariate analysis used to study the classification of these relay cells in the cat LGN. The receptive-field properties of 135 LGN relay cells have been studied in cats paralyzed and anesthetized with N2O/O2 (70/30%). A new technique, the spatiotemporal map (S-T map), has been developed. This technique provides a visualization of the temporal pattern of a cell's response to a small spot flashed on and off in 50 successive overlapping positions across the receptive field. A second technique using a centrally located flashing spot of increasing diameter has been employed to examine the inhibitory effect of the surrounding region on the response of the cell. Using these two techniques on the same cells in the LGN, it has been found that the receptive fields of X- and IM-cells show a classical center-surround organization. Conversely, the receptive fields of Y-cells are not organized in a simple center-surround fashion. They are composed of three regions: one central region of center-type response surrounded by a region of mixed center-type and surround-type responses and, more peripherally, by a region of surround-type response that shows an inhibitory effect on the center response. The inhibitory strength of the surrounding region is stronger for X-cells than for IM-cells, and stronger for IM-cells than for Y-cells. Furthermore, for all cell classes, the inhibitory strength of the surrounding region is a decreasing function of the receptive-field center size. This may account for the differing responses of X- and IM-cells to visual stimuli. Although they share the same receptive-field organization as X-cells, IM-cells have larger center sizes and weaker surrounds, and thus respond better to large targets rapidly moved across their receptive fields and to large spots flashed on and off in their receptive fields. The linearity of response has been studied in X- and IM-, and Y-cells using a contrast-reversal pattern. X- and IM-cells exhibited linear summation within their receptive fields, whereas Y-cells did not. A multivariate-analysis technique has been applied to the quantified measures obtained on X-, IM-, and Y-cells. The results of this analysis and the study of the receptive-field properties of X-, IM-, and Y-cells lead us to conclude that IM-cells are, in fact, members of the X-cell class, IM-cells being X-cells that have larger receptive-field centers and, consequently, weaker inhibitory surrounds. We also conclude that Y-cells constitute a separate cell class and that there does not appear to be a continuum of properties between the X and Y classes. Our results support the hypothesis that parallel X and Y channels transmit information from the retina through the lateral geniculate nucleus to the visual cortex.