1. Receptive-field properties of single neurons in cat's cortical area 18 were studied before and after partial bilateral lesions of area 17. 2. The majority of cells recorded from animals with intact visual cortex exhibited orientation selectivity, directional selectivity, and could be independently activated through either eye. All cells responded well to moving targets and nearly all of them exhibited broadly tuned preferences with respect to speed of the target. Over 45% of cells responded optimally or exclusively at very fast (above 50 degrees/s) speeds. 3. The majority of neurons recorded from animals with intact visual cortex responded weakly but clearly to appropriately oriented localized stationary stimuli flashed on and off. About one-third of the cells responded with mixed on-off discharges from all over their receptive field. In the receptive fields of 10% of cells, separate on- and off-discharge regions could be revealed. In the receptive fields of the remaining cells, only on- or only off-discharge regions could be revealed. 4. The majority of neurons recorded after ablation of area 17 were orientation selective; 50% of the cells were also direction selective. All neurons responded well to moving targets; about 65% of them responded optimally or exclusively at very fast target speeds. 5. Destruction of the dorsolateral part of contralaterial area 17 and most of contralateral area 18 caused significant reduction in proportion of cells in area 18 which could be activated through either eye. 6. The majority of neurons recorded after ablation responded to appropriately oriented localized stationary stimuli flashed on and off. Cells with mixed on-off discharge regions all over the receptive field with separate on- and off-discharge regions and with only on- or only off-discharge regions were found. 7. It is concluded that the processing of afferent visual information in area 18 is, to a great extent, independent of the information carried to this area by associational fibers from cells of area 17.