Northern/slot blot analysis was used to determine postnatal developmental and environmentally induced changes in the level of expression of GAP-43 mRNA in visual and frontal cortex. Both structures showed a precipitous decline during the first 5 weeks and a slight further decline to adult levels. Dark rearing resulted in a significant elevation of GAP-43 mRNA which was eliminated by brief visual experience. This effect was specific to visual cortex and did not occur in frontal cortex. The effect also did not occur in normal adult cats placed in prolonged darkness, indicating that GAP-43 mRNA levels are not simply activity dependent and are altered by visual input only during early postnatal life. These results are consistent with a role for GAP-43 in the state of visual cortical plasticity.