Spectral, spatial and temporal properties of receptive fields of dark-adapted, on-off retinal ganglion cells in the intact eye of the plaice, were analysed by recording from their axon terminals in the superficial layers of the optic tectum with indium micro-electrodes.2. Two cell-types were identified. The first gave fast-adapting, spectrally opponent on-off responses without centre-surround subdivisions of the receptive field. On and off response-components were mutually antagonistic. The second type gave slow-adapting on-off or off responses for different stimulus positions within the receptive field, with centre-surround or adjacent field configurations. Only on-off centre cells, showing mutual antagonism between field centre and surround, or off centre cells with inhibitory centres, were found. These cells had weak opponent or non-opponent properties.3. Most cells of each type received inputs both from cones and rods. At stimulus intensities suprathreshold for cones, response-components gave spectral peaks which have been classified into one of four wave-length ranges; blue, 440-460 nm; blue-green, 470-490 nm; green, 510-540 nm; and orange, 560-590 nm. No cells analysed gave sensitivity maxima in the red. At low stimulus intensities all cells with rod input gave a single spectral peak between 510 and 530 nm.
