LONG-TERM potentiation (LTP) is a lasting enhancement of excitatory synaptic transmission that follows specific patterns of electrical stimulation(1). Although the mechanism of LTP has been intensively studied, particularly in the hippocampus, its significance for normal brain function remains unproven. It has been proposed that LTP-like mechanisms may contribute to naturally occurring, experience-dependent synaptic modifications in the visual cortex(2-8). The formation of normal binocular connections within the, visual cortex : requires simultaneous input from both eyes during a postnatal critical period(9-12) that can be delayed by rearing animals in complete darkness(13,14). To explore the role of LTP in this experience-dependent maturation process, we induced LTP in visual cortical slices taken at different ages from light-reared and dark-reared rats. Susceptibility to LTP coincides with the critical period and, like the critical period, can be prolonged by rearing animals in darkness. These findings support the hypothesis that LTP reflects a normal mechanism of experience-dependent synaptic modification in the developing mammalian brain.