The orientation selectivity of cells in ferret primary visual cortex was studied during normal development and in animals deprived of vision or of visual cortical activity. In normal animals from the age when visual responses were first recorded (postnatal day 23) through postnatal week 5, only about 25% of cells showed orientation-selective responses. By postnatal week 7, cortical responses had matured to an adult-like state, with approximately 75% of cells clearly selective for orientation. This development of orientation selectivity was not merely a reflection of the development of cortical cell responsiveness: at all ages studied, there was no correlation between responsiveness and selectivity. Infusion of TTX into visual cortex to silence neuronal activity completely blocked the maturation of orientation selectivity. Visual deprivation by bilateral lid suture impaired but did not completely block the normal development of orientation selectivity. We conclude that the maturation of orientation-selective responses in ferret primary visual cortex requires cortical neuronal activity, and that normal development requires visually driven activity.