Quantitative estimations of cortical complexity are used in the clinic as a measure of consciousness levels, but the cortical mechanisms involved are poorly understood. We used a version of the Perturbational Complexity Index adapted to the cerebral cortex in vitro (sPCI) to investigate the role of gabaergic inhibition in cortical complexity. We studied two dynamical states: slow-wave activity and “awake-like” activity, which express low and high causal complexity respectively. Progressive blockade of GABAergic inhibition during both regimes revealed its impact on emergent cortical activity and on sPCI. Gradual GABAA-Rs blockade resulted in higher synchronization and progressively reduced sPCI. Blocking GABAB-Rs also resulted in a reduced sPCI while in slow-waves. Our findings demonstrate that physiological levels of inhibition contribute to generation of dynamical richness and cortical complexity. However, if inhibition is diminished or enhanced, cortical complexity decreases. Using a computational model we demonstrate a link between excitatory/inhibitory balance and cortical complexity.