Mutations in the presynaptic adhesion protein Neurexin-1 (NRXN1) have been repeatedly implicated in cases of autism, schizophrenia, and ADHD. The excitation to inhibition (E:I) imbalance hypothesis posits that behaviours associated with neurodevelopmental disorders such as autism may be driven by the disturbed balance of excitation and inhibition in the cortex. We aimed to identify and characterise the effect of Neurexin-1 mutations on the E:I balance in autism-associated cortical circuits.To assess the effect of Nrxn1 alpha KO on synaptic transmission, whole-cell electrophysiological recordings were obtained from layer 5 pyramidal neurons in medial prefrontal cortex (mPFC) acute brain slices from postnatal day 19-21 Nrxn1 alpha KO and WT littermates. Recorded neurons were filled with biocytin and putative excitatory and inhibitory synapses contacting them were detected via immunohistochemistry.Nrxn1α KO neurons exhibited an increased frequency but not amplitude of miniature inhibitory postsynaptic currents (mIPSCs) when compared to WT, combined with unaltered frequency or amplitude of excitatory mPSCs. The density of inhibitory and excitatory synapses did not differ between Nrxn1α KO and WT neurons. Nrxn1 alpha KO neurons exhibited a lowered inhibitory paired pulse ratio.Taken together, these results indicate an increase in inhibitory synaptic transmission in Nrxn1α KO mPFC paired with unaltered excitatory transmission. This appears to be driven by an increase in inhibitory synapse release probability, rather than a change in the sheer number of inhibitory synapses. These findings directly implicate Nrxn1 mutation in the emergence of E:I imbalance in ASD-relevant cortical circuits.