The influence of iontophoretically applied acetylcholine (ACh) on single-unit activity in the visual cortex was studied in anesthetized cats. The dominant effect consisted of a slow facilitation of neuronal responses to moving light bars. This cholinergic action was sometimes paralleled by a decrease of the cells' selectivity to the direction of stimulus movement. In about one-third of the neurons studied ACh-iontophoresis suppressed maintained and visually driven activity within a few hundred milliseconds from the onset of its application. This effect was antagonized by concurrent iontophoresis of the muscarinic ACh-receptor antagonist scopolamine and the GABAA-receptor antagonist bicuculline methiodide. In 7% of the units studied a fast excitation was elicited by ACh application showing a similar time course as the rapid suppressive effect. It is concluded that ACh-induced inhibition is mediated by an activation of GABAergic interneurons. The role of cholinergic depression and facilitation in cortical information processing is discussed.