Weak inhibition within visual cortex early in life prevents experience-dependent plasticity. Loss of responsiveness to an eye deprived of vision can be initiated prematurely by enhancing γ-aminobutyric acid (GABA)–mediated transmission with benzodiazepines. Here, we use a mouse “knockin” mutation to α subunits that renders individual GABA type A (GABA A ) receptors insensitive to diazepam to show that a particular inhibitory network controls expression of the critical period. Only α1-containing circuits were found to drive cortical plasticity, whereas α2-enriched connections separately regulated neuronal firing. This dissociation carries implications for models of brain development and the safe design of benzodiazepines for use in infants.