1. The spectral, spatial and temporal properties of receptive fields of dark-adapted, on-off retinal ganglion cells in the intact eye of the plaice, have been analysed by recording responses from their axon terminals in the superficial layers of the contra-lateral optic tectum with indium micro-electrodes. 2 Two cell-types have been identified on criteria of discharge patterns. The first type gives spectrally opponent "on-off" responses to coloured stimuli, with no subdivision of receptive fields into centre and periphery. On and "off" response-components are mutually inhibitory. The second type gives slow-adapting, "on-off" or "off" responses for different stimulus positions within the receptive field, with centre-surround or adjacent field patterns. Only "on-off" centre, off"-surround cells, or "off"-centre, "on-off" surround cells have been found, "On-off" centre cells exhibit mutual antagonism between field centre and surround. "Off"-centre cells possess inhibitory centres. This cell-type gives only weak opponent, or possibly non-opponent responses. 3. Moot cells of each type receive rod input in addition to input from cones. At stimulus intensities suprathreshold for cones, response-components give spectral maxima or more of four wavelength ranges; blue, 440-460 mum; blue-green, 470-490 mμ; green, 510-540 mμ; and orange, 560-590 mμ. No cello give red sensitivity maxima. At low stimulus intensities all cells with rod input give a single spectral peak between 520 and 530 mμ.