1. This investigation is based upon Alpern's (1965) contrast flash observations. The threshold for the test flash lambda (Fig. 2a) is raised if a second flash varphi falls on the annular surround. Moreover, if lambda excites rods at threshold, it is only the rods in the surround that contribute to the threshold rise.2. The possibility that the rise in lambda threshold might be due to light physically scattered from surround to centre we exclude by several different experiments. We conclude (Fig. 1b) that the varphi flash sets up a nerve signal N which is conducted to some place C where it inhibits the signal from the centre.3. If the luminous surround, instead of being a full circle (Fig. 2a) consists only of the sectors shown black in Fig. 2b, that occupy 1/m of the surround area, it is found (in the physiological range) that the light/area on those sectors must be m times as great to produce the same threshold rise at centre, i.e. the total surround illumination must remain the same.4. This result would obviously follow if N, the inhibitory nerve signal, were proportional to the total surround illumination. We have established the converse; the signal must be proportional to the quantum catch.5. Light can be increased indefinitely, nerve signals cannot. When varphi increases sufficiently, N saturates in the same way that S-potentials and receptor potentials saturate, namely according to N = varphi/(varphi + sigma) where sigma, the semi-saturation constant is about 200 td sec, or 800 quanta absorbed per rod per flash.6. Thus the nerve signal N is proportional to the quantum catch over 4 log units in the physiological range, namely from 1 quantum per 100 rods to 100 quanta per rod per flash. Above this for another 2 log units N continues to increase, but now more slowly, after the manner of S-potentials and receptor potentials.