A failure of integrative processes within the brain, mediated via altered GABAergic inhibition, may underlie several features of schizophrenia. The present study examined, therefore, whether maternal immune activation (MIA), a risk factor for schizophrenia, altered inhibitory markers in the hippocampus and medial prefrontal cortex (mPFC), while also altering electroencephalogram (EEG) coherence between these regions. Pregnant rats were treated with saline or polyinosinic:polycytidylic acid mid-gestation. EEG depth recordings were made from the dorsal and ventral hippocampus and mPFC of male adult offspring. Glutamic decarboxylase (GAD67) levels were separately assayed in these regions using western blot. GAD67 expression was also assessed within parvalbumin-positive cells in the dorsal and ventral hippocampus using immunofluorescence alongside stereological analysis of parvalbumin-positive cell numbers. EEG coherence was reduced between the dorsal hippocampus and mPFC, but not the ventral hippocampus and mPFC, in MIA animals. Western blot and immunofluorescence analyses revealed that GAD67 expression within parvalbumin-positive cells was also reduced in the dorsal hippocampus relative to ventral hippocampus in MIA animals when compared with controls. This reduction was observed in the absence of parvalbumin-positive neuronal loss. Overall, MIA produced a selective reduction in EEG coherence between the dorsal hippocampus and mPFC that was paralleled by a similarly specific reduction in GAD67 within parvalbumin-positive cells of the dorsal hippocampus. These results suggest a link between altered inhibitory mechanisms and synchrony and, therefore point to potential mechanisms via which a disruption in neurodevelopmental processes might lead to pathophysiology associated with schizophrenia.