Visual cortical neurons selectively respond to particular features of visual stimuli. Selective visual responsiveness is modified by visual expe- rience during development. We report that fine-scale networks of precisely interconnected excitatory neurons were embedded in the rat visual cortex and suggest that this network could be a functional unit for visual information processing. We also investigated the effects of visual dep- rivation on the development of visual cortical circuits. We used two kinds of deprivation, binocular deprivation and dark rearipg, which allowed visual inputs with only diffuse light and no visual input, respectively. The probability and strength of excitatory connections t layer 2/3 pyrami- dal cells increased during the 2 weeks after eye opening, and these changes were prevented by dark rearing, but not binocular eprivation. Fine- scale networks were absent just after eye opening and established during the following 2 weeks in rats reared with normal visual experience, but not with either type of deprivation. These results indicate that patterned vision is required for the emergence of the fine-scale network, whereas diffuse light stimulation is sufficient for the mituration of individual synapses. The critical role of early sensory experience may be to organize cell assemblies underlying visual information processing in the visual cortex.