1. The responses to visual stimuli of cells in the dorsal lateral geniculate nucleus of the rat were recorded with micro-electrodes. 2. Maps made with small spots of light showed that most units had concentrically organized receptive fields. Some units gave 'on-off' responses to spots flashed anywhere within the receptive field. These units were not directionally selective. 3. By the use of grating patterns as stimuli, units with concentrically organized receptive fields could be divided into groups that showed linear or non-linear spatial summation. Those unit showing linear spatial summation behaved like the 'X' cells of the cat, those showing non-linear summation like 'Y' cells. 4. 'On-off' units showed non-linear spatial summation of a kind that readily distinguished them from Y cells. 5. Measurements of spatial contrast sensitivity made with moving gratings showed, for both X and Y cells, peak sensitivities for spatial frequencies between 0.05 and 0.09 c/deg X and Y cells were not distinguished by their preferred spatial frequencies at any eccentricity.