The receptive field properties and responses to electrical stimulation of 126 P-cells recorded from the dorsal lateral geniculate nucleus (LGNd) were studied in the hooded rat. 2. Eighty-five cells had a concentric (Kuffler, 1953) receptive field organisation (46 off-centre on-surround; 39 on-centre off-surround). Of the remaining cells 29 had co-extensive on/off excitatory discharge regions, nine had on-centres with suppressive surrounds and two cells gave on-responses but had no suppressive surround. One cell was identified as suppressed-by-contrast. 3. On the basis of the battery of tests developed for the identification of cell types in the cat's retina and LGNd, 35 of the cells with a Kuffler-type receptive field organisation were identified as Y-like. The majority of the remaining cells, both concentric and others, reminded us of the different subclasses of W-cells of the cat. Nine concentric cells in most of the tests exhibited X-like properties. 4. All of the Y-like cells were driven by relatively fast conducting retinal ganglion cell axons, comprising the t1 conduction velocity group. The majority of the remaining cells were driven by slower axons comprising t2 or t3 conduction velocity groups. 5. Thus, in the rat, as in other mammalian species studied so far, there is a correlation between the conduction velocity groups in the retino-geniculo-cortical pathway and the functional groups based on the cells' receptive field properties. There seem to be functional equivalents of the cat's Y- and W-cell classes but evidence for a distinct X-like class of cells is lacking.
