Measurements were made of the range of threshold (dl) in response to a blue (451 nm) and a green (550 nm) monochromatic test light during exposure to step-wise increased backgrounds of illumination Photopic vision (I) in man. Criterion was the increment of radiation power of a test stimulus necessary for a threshold sensation and for a constant latency of the negativity N<sub>2</sub> of the visually evoked cortical potential (VECP). In darkness and at low background illumination (below log –0.70 td) latencies were linearly related to the radiation power of the test light between 200 and 280 msec. At higher levels of illumination a similar relation was seen for latencies between 125 and 200 msec. Within the low and the high range of background illumination dl/I curves were obtained for a criterion latency of 240 msec (low background illumination) and of 170 msec (high background illumination). At background illumination above the transition point (log ––0.70 td) the change of threshold for blue was greater than for the green test stimulus indicating a Purkinje shift in the VECP. For the sensory measurement the dl/I curves for the blue and green test stimuli were seen to split into branches at background illumination of log ––0.70 td, i.e. at the transition point between the scotopic and the photopic range of latencies of the VECP. It is concluded that the determination of incremental threshold is a useful means of separating scotopic and photopic activities of the visual system for physiological and clinical purposes.