In this study we investigated the functional implications of GABA A receptor α1 subunit deletion on dendritic arborization and spine maturation in the visual cortex. This subunit is normally strongly upregulated during early postnatal development. Our main finding is that mice lacking the GABAA receptor α1 subunit displayed an increased density of dendritic filopodia during the second and third postnatal weeks. However, there was a concomitant decreased density of mature mushroom-shaped spines, which became more pronounced in adults. In contrast, dendritic arborization was not altered in these mice. We propose that an increased efficacy of the inhibitory synaptic transmission in the α1 knock out mice may lead to an enhancement of the outgrowth of filopodia around eye opening, but to a failure in spine maturation at later stages. © 2003 IBRO. Published by Elsevier Ltd. All rights reserved.