We have examined the effects of iontophoresing specific antagonists to excitatory amino acid receptors on the visual responses of cells in lamina A or A1 of the cat's lateral geniculate nucleus (LGN). Cells were classified as ON- or OFF-center, X or Y, and lagged or nonlagged. The effects of antagonists were studied while cells were stimulated with spots of the appropriate contrast covering the receptive-field center. The N-methyl-D-aspartate (NMDA) receptor antagonists D-2-amino-5-phosphonovaleric acid (D-APV) and 3-((±)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP), when iontophoresed at doses that specifically antagonized NMDA-induced responses but not kainate-induced responses, reduced the responses of all cell types in the LGN, including X and Y cells, lagged and nonlagged cells, and ON- and OFF-center cells. The non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), when applied at doses that specifically antagonized kainate-induced responses but not NMDA-induced responses, also reduced the visual responses of each of the cell types in the LGN. We analyzed quantitatively the effects of D-APV and CNQX on LGN cells. D-APV reduced the responses of lagged cells to a greater extent than the responses of nonlagged cells. CNQX reduced the responses of lagged and nonlagged cells to a similar extent. There was no difference in the effect of D-APV or of CNQX on X and Y cells or on ON- and OFF-center cells. We analyzed the effects of the antagonists on separate components of responses, including an early component comprising the first 100 ms of response and a late component comprising the next 300 ms of response. D-APV reduced the late component of lagged cell responses to a greater extent than either the early component of the same cells or the early or late component of nonlagged cells. CNQX had nearly equivalent effects on both response components of all cell types. These data indicate that NMDA and non-NMDA receptors make similar contributions to the responses of ON- and OFF-center and X and Y cells in the LGN. Lagged and nonlagged cells are not differentiated with respect to the contribution of non-NMDA receptors to their visual responses. The greater contribution of NMDA receptors to the responses of lagged cells is consistent with the large contribution made by these receptors to the late response components of lagged cells. It is possible, then, that the degree of participation of NMDA receptors in the visual responses of LGN cells is a function of the response characteristics of cells rather than a fundamental distinguishing feature of LGN cell types.