Experimental evidence derived from in vitro studies suggests that synaptic plasticity may be involved in information processing in the adult sensory cortex. Here, we demonstrate, for the first time, that synaptic plasticity that endures for >24 h can be elicited in the visual cortex of freely moving adult rats. Both long-term potentiation (LTP) and long-term depression (LTD) were evoked in layer 2/3 of the primary visual cortex after stimulation of the dorsal lateral geniculate nucleus. We found that synaptic plasticity within visual cortex synapses is not a static phenomenon, where a particular pattern of stimulation invariably evokes the same form of synaptic plasticity but rather fluctuates in association with diurnal luminance levels. Whereas acute (12 h) dark exposure drives intrinsic synaptic transmission to basal levels, light exposure (12 h) leads to synaptic potentiation. Furthermore, low-frequency stimulation at 3 Hz generates LTD in light-exposed animals, and LTP in dark-exposed animals, in close correlation with the time course of natural slow wave oscillations. Our data demonstrate that the adult visual cortex is in a perpetually dynamic state, where the direction of plasticity changes depends on the immediate visual experience.