FAST associative cellular plasticity of visual cortical cells has been shownin vitrowith electrical stimulation as well asin vivoby pairing of natural stimuli with artificial depolarization. Here we experimentally changed receptive field properties of single cells in the mature cat visual cortex with purely natural associative stimulationin vivo. Single cortical cells expanded their receptive fields within minutes into unresponsive regions and changed their functional receptive field structure for hours after associative co-stimulation of active and primarily unresponsive regions. The effects are interpreted as strengthening of subthreshold synaptic inputs. Repetitive stimulation with the same visual pattern can modify single cell propertiesin vivowithin minutes and may be related to preattentive fast perceptual learning.