Functional characteristics of 103 neurons in field 17 of the visual cortex were studied in immobilized and unanesthetized cats. Unit responses to modification of local light stimuli wer recorded under conditions of light adaptation. Mean photic threshold of investigated neurons was -32 dB, mean critical temporal summation 57 ms, duration of reactivity recovery 190 ms and sharpness of orientation tuning 37 degrees. As compared to dark adaptation, under light adaptation reduction of light sensitivity, sharpening of orientation tuning as well as a lengthening of temporal summation and a shortening in time of reactivity recovery were observed. A number of neuronal characteristics depended on excentricity of their receptive fields: cells with centrally localized receptive fields had a lower threshold, a short summation and rapid recovery of reactivity; their activity was of higher frequency, and they generated brief transient responses more often than cells with receptive fields in the periphery of a visual field. Mechanisms responsible for change of neuronal properties in the central and peripheral visual channels under light and dark adaptation are discussed. Several inhibitory subsystems are suggested by means of which neuronal activity of the visual cortex is regulated.