Focal cortical dysplasia type ll (FCD2) is a congenital brain abnormality characterized by atypical focal brain organizations and dysmorphic neurons, frequently associated with refractory epilepsy. One causative mutation of FCD2 is a gain-of-function mutation in Ras Homologue Enriched in Brain (RHEB), RHEB-p.P37L. Studies on tissue resected from FCD2 patients suggest an elevated NKCC1:KCC2 compared to control tissue. The relative expression of these chloride transporters regulates the chloride reversal potential (ECl). The typical hyperpolarization of ECl during development, due to the upregulation of the Cl--extruder KCC2, ensures that GABA gains its inhibitory polarity. If the immature expression pattern of NKCC1:KCC2 in FCD2 shifts GABA’s polarity towards depolarization, this may contribute to epilepsy development in patients. This study explores whether the expression of RHEB-p.P37L in murine hippocampal neuronal networks cultured on a multi-electrode array (MEA) leads to altered network activity and changes in the response to blocking of GABAergic signaling. Intrinsic network activity of RHEB-p.P37L cultures was altered, showing faster network synchronization, increased bursting activity, and altered burst characteristics. Interestingly, recordings of network responses to the GABAA-channel blocker picrotoxin showed that, in contrast to increased network activity in control cultures, network activity in RHEB-p.P37L cultures decreased when GABAergic transmission was blocked. These results support the hypothesis that ECl is more depolarized in RHEBp.P37L-expressing neurons, which could promote the hyperexcitability of the network and thereby contribute to the development of epilepsy.