In the present investigation the effects evoked by prolonged ad aptation at different light backgrounds have been studied on the maintained discharge of single neurons in the striate area 17 and non-striate areas 18 and 19 of the cat cerebral cortex. The animals were unanaesthetized, Flaxedil paralyzed and artificially ventilated under continuous control of the CO₂ expired. Two or more of four fairly different light backgrounds, corresponding to $2 \cdot 10^{-3}$, $4 \cdot 10^{-2}$, $9 \cdot 10^{-2}$ and $2 \cdot 10^{-1}$ cd/m², were presented in random sequences to the cat eyes. The adaptation time for each luminous level was 45-60 min; the greater the difference in light background between two consecutive levels, the longer was the adaptation time. The analysis of the data obtained from the cortical neurons showed that the mean frequency of the discharge was directly related to changes in the level of the light background for striate neurons, whereas it was inversely related for non-striate neurons.