The cortex receives a cholinergic input which is considered to be involved in mediating the effects of arousal. The experiments reported here have examined the nature of the cholinergic influence on the neuronal organization of the cat visual cortex. Out of 83 cells studied, 92% exhibited a modification in their visual response properties during the iontophoretic application of ACh. These comprised 61% in which responses were facilitated and 31% in which responses were depressed. The facilitatory effects were associated with a striking increase in stimulus specific responses without any concomitant loss in the selectivity. This comment applied equally to orientation and direction selectivity. It is argued that the facilitatory action of ACh on stimulus specific responses is consistent with a modulation of potassium conductance and most probably the conductance associated with a voltage dependent channel. We found no evidence to support the view that the facilitatory action involved disinhibition; the action of bicuculline, which blocks inhibitory influences in the visual cortex, was quite distinct to that of ACh. The facilitatory and depressive effects of ACh did not show any correlation with the simple-complex classification of cells or any other obvious parameter of receptive field organization, but there was a correlation with cortical lamination. Cells facilitated by ACh were found in all cortical laminae, but those depressed by ACh were found in laminae III and IV.