This paper describes the behaviour of the receptive field surround, and how surround signals combine with those from the centre to generate the discharge of the retinal ganglion cells of the cat. 2. A small test spot is flashed upon the middle of the receptive field of an on-centre X-cell, alone, or together with a concentric annulus of fixed luminance. The reduction in discharge brought about by the annulus is independent of spot luminance. From this it is inferred that centre and surround signals combine additively. 3. Knowing that the combination of signals is additive, the surround signal can be estimated by comparing the ganglion cell's response to diffuse illumination of its receptive field with that to an equiluminous spot which optimally stimulates the centre while encroaching minimally upon the periphery. 4. Application of this technique to X-cells shows that although the surround seems to have a threshold, it is at its most sensitive in the dark-adapted eye, and typically is only 0.3-0.5 log units less sensitive than the centre. 5. Centre and surround sensitivities are decreased from their dark-adapted levels by increasing background illumination, but the decline of surround sensitivity is initially less rapid than that of the centre. Thus with increasing light-adaptation the surround becomes relatively more sensitive. In the light-adapted eye centre and surround are about equally sensitive to diffuse illumination. 6. Although, in the dark-adapted eye, illumination of the receptive field periphery of an on-centre unit depresses firing, removal of that illumination produces no off-discharge. Off-discharges appear only when background illumination exceeds about 104 quanta (507)/deg 2 sec. This confirms Barlow & Levick (1969b). 7. In the dark-adapted eye surround latency is longer than that of the centre. With increasing background illumination the latency difference is reduced. 8. For X-cells, the rate of the maintained discharge depends to some extent on the balance of centre-surround antagonism. But this antagonism is not the major factor accounting for the relative constancy of mean rate at high background luminances, for the rate then can be almost independent of the size of a steady pot. 9. The mean rate of discharge of Y-cells seems to depend even less upon the balance of centre-surround antagonism. 10. Y-cell surrounds could not properly be isolated with the optimal spot-diffuse illumination technique, so detailed measurements of their behaviour were not made. However, the dark-adapted surround appear to be as sensitive as those of X-cells.
