Deprivation of visual experience for a short time during the early period postemergence in the fly influences strongly the development of visual pattern discrimination. Cytochrome oxidase histochemistry was used to examine the effect of visual deprivation on the development of the neuronal network in the optic lobe. Flies in which one compound eye was covered immediately after emergence were raised under normal light and dark conditions (LD). Other flies were raised without covering of the compound eyes under LD or continuous darkness (DD) as controls. At various periods of postemergence flies from the 3 groups were used for histochemistry. The results indicated that the deprivation caused by unilateral eye covering produced a decrease in cytochrome oxidase staining of the covered side of the retina and optic lobe, but not in the central part of the brain. This asymmetry in staining was not seen in flies of 1-5 days postemergence but became evident after the sixth day postemergence. When flies were raised in DD until the 5th day postemergence and then in LD, the development of asymmetry in cytochrome oxidase staining was delayed. These results confirm previous behavioral experiments on visual deprivation. Both suggest that visual deprivation during the early period of postemergence leads to a long-lasting decrease in neuronal activity.