While in-vitro experiments highlight the specific role of GABAergic neurons to neonatal network dynamics, their contribution to in-vivo activity remains elusive. To address this issue we developed an in-vivo approach combining optogenetics and single-unit electrophysiology in non-anesthetized, head restrained mice. This allowed us to both identify and manipulate GABAergic cells while examining their influence on the spontaneous network activity. The results will be presented in the context of excitation/inhibition balance in the immature brain.