In the cat, lateral geniculate neurons and retinal ganglion cells can almost all be categorized as on-center (excited by light stimuli) or off-center (excited by dark stimuli). We have investigated how these cells are affected when a drug is applied to the retina that, in the mudpuppy, inactivates on-bipolar cells (Slaughter, M. M., and R. F. Miller (1981) Science 211: 182–185). This drug, D,L-2-amino-4- phosphonobutyric acid (APB), is an analogue of glutamate. After injecting APB into the vitreous of the eye, we could record in the optic tract from fibers of off-center retinal ganglion cells that appeared normal in their responses, but we could find no fibers from on- center cells in the injected eye. In the lateral geniculate nucleus (LGN), the outcome was similar. Of a sample of 144 cells studied quantitatively, 13 had extremely poor responses and could not be characterized as on-or off-center. The remaining 131 cells had off- centers and appeared largely normal. Their receptive field organization was unaltered: the field center was excited by dark (off) stimuli and inhibited by light, whereas the surround was inhibited by off stimulation. Of particular interest was the fact that the surrounds of many off-center cells could still be excited by light stimuli; thus, despite the probable inactivation of on-bipolar cells, not all on- responses in the LGN were abolished. As a group, off-center cells were somewhat less responsive then in control experiments. Except for this change, APB appeared to affect only on-center cells.(ABSTRACT TRUNCATED AT 250 WORDS)