1. The properties of the surround response mechanism of on-centre cells and its interaction with the centre mechanism were studied by recording from single optic tract fibres. In many of the experiments the spatial distribution of the light within the retinal image of the stimuli was measured.2. Pure surround responses of on-centre cells were isolated using a centrally located steady light which selectively desensitized (adapted) the centre mechanism. This permitted a peripheral flashing stimulus whose luminance varied over a range as great as 1.38 log units to elicit surround responses which, for any given cell, were of invariant shape. The rate of decay of the firing frequency of the spike burst at ;off' varied from cell to cell. The general characteristics of such pure surround responses to squarewave stimuli were described. The plot of the magnitude of pure responses against stimulus luminance, at constant background conditions, was curvilinear.3. The pure surround response of two off-centre cells was isolated; it was similar in shape to the pure centre response of on-centre cells.4. Interaction of centre and surround mechanisms of on-centre cells was studied by eliciting a pure central and a pure surround response from the same cell. The electronically obtained algebraic sum of these two pure responses equalled the mixed response of the ganglion cell to simultaneous presentation of the stimuli which evoked the pure responses when presented singly. This is probably best explained by algebraic summation of centre and surround inputs.5. The pure surround response from two cells to a fixed flashing stimulus was attenuated by a steady field adapting light, both when this was superimposed upon the stimulus and when not superimposed. In the latter case, (i) when the spatial separation between the flashing stimulus and the adapting light was at a minimum, less than 10% of the adapting flux fell inside the boundaries of the stimulating flux, and (ii) the response was attenuated also if the adapting light was in the geometric centre of the receptive field. These results indicate that the adaptation pool of the surround mechanism extends to the central portions of the receptive field.6. Nearly half the cells tested did not yield pure surround responses. This was probably due to differences, within the ganglion cell population, (i) of the spatial distribution of the ratio of centre to surround signal sensitivity and (ii) of differences in the ratio of centre to surround adaptivity in the receptive field middle. It was not due to excess adaptive flux falling outside the region of maximal centre mechanism adaptivity, nor due to excess stimulus flux falling inside the region of maximal signal sensitivity.