Abstract The influence of ambient illumination on the maintained electrical activity of single neurons of the cat visual cortex was studied by using the closed chamber technique for extracellular recordings. Several levels of light background within the scotopic-mesopic range were explored. Phasic and tonic changes in firing rate were observed following a background change. The former were irregular and unpredictable variations lasting up to 15–20 min. The latter, which usually followed the phasic changes, showed the constant characteristic of being in direct relation to luminance variations for neurons isolated in the striate area and in inverse relation for units recorded from the two non-striate areas of the visual cortex; in all cases, they lasted until a new luminous level was set. Changes in firing rate were not dependent upon either the neuron receptive field organization or the EEG pattern, simultaneously recorded. The background-locked firing rate variations recorded at the visual cortex seem to be the result of a particular cortical distribution of afferent fibers carrying luminance information. Applications to vision research are also suggested.