Spatial and temporal modulation transfer functions have been measured as a function of luminance at scotopic and mesopic levels. It is found that throughout the scotopic range the data can be accounted for by a two process visual system, with the processes having the following properties. Simple excitatory processThis mechanism summates over somewhat less than 1 square degree of visual angle and over about 200 msec of time. These properties do not change with luminance, but the over-all sensitivity of the mechanism follows the De Vries-Rose law. The temporal properties of this mechanism follows those of the scotopic b-wave. At high luminances, the temporal, but not spatial, properties of this mechanism break down in a manner which had not been studied.Low-frequency inhibitory processThis process is manifest as a decrease in sensitivity from that of the simple excitatory process. Its effect is observed only when both spatial and temporal frequency are low, suggesting that it has a larger (perhaps 3 x) summation area in both space and time. The inhibitory process is not observed below a certain threshold luminance, which is highly dependent upon the configuration of the stimulus. For a suitable stimulus, this threshold will be well into the scotopic luminance range.It is suggested that these two processes represent psychophysical correlates of the centre and surround of retinal receptive fields.