We examined the mechanisms through which the prolonged presentation of either a high-contrast stimulus or an artificial scotoma [equivalent to the stimulation of the receptive field (RF) surround] induces changes in the RF properties of neurons intracellularly recorded in cat primary visual cortex. Discharge and synaptic RFs were quantitatively characterized using bright and dark bars randomly flashed in various positions. Compared with the lack of stimulation (0% contrast for 15-30 s), stimulation with high-contrast sine-wave gratings (15-30 s) was followed by a strong reduction in gain and a weak but significant reduction in width of spike discharge RFs. These reductions were accompanied by a membrane potential hyperpolarization, a decrease of synaptic RF width, and varying changes of synaptic RF gain. Passive hyperpolarization by DC injection also produced significant reduction in the width and gain of discharge RF. Mimicking, in single neurons, high-contrast stimulation with high-intensity current injection also induced a membrane potential hyperpolarization, whose amplitude was correlated with discharge RF gain and width changes. Recovery from adaptation to high-contrast stimulation during the period of gray screen or scotoma presentation was associated with an increase in gain and discharge RF size. Stimulation of the RF surround with an artificial scotoma did not have any additional aftereffects over those of adaptation to a gray screen, indicating that the contraction and expansion of RF gain and size are attributable to intrinsic and synaptic mechanisms underlying adaptation and de-adaptation to strong visual stimuli.