Noncompetitive N-methyl-d-aspartate receptor (NMDAr) antagonists can elicit many of the symptoms observed in schizophrenia in healthy humans, and induce a behavioural phenotype in animals relevant to psychosis. These compounds also elevate the power and synchrony of gamma (γ) frequency (30-80 Hz) neural oscillations. Acute doses of antipsychotic medications have been shown to reduce ongoing γ power and to inhibit NMDAr antagonist-mediated psychosis-like behaviour in rodents. This study aimed to investigate how a chronic antipsychotic dosing regimen affects ongoing cortical γ oscillations, and the electrophysiological and behavioural responses induced by the NMDAr antagonist ketamine. Male Wistar rats were chronically treated with haloperidol (0.25 mg/kg/d), clozapine (5 mg/kg/d), LY379268 (0.3 mg/kg/d) or vehicle for 28 d, delivered by subcutaneous (s.c.) osmotic pumps. Weekly electrocorticogram (ECoG) recordings were acquired. On day 26, ketamine (5 mg/kg, s.c.) was administered, and ECoG and locomotor activity were simultaneously measured. These results were compared with data generated previously following acute treatment with these antipsychotics. Sustained and significant decreases in ongoing γ power were observed during chronic administration of haloperidol (64%) or clozapine (43%), but not of LY379268 (2% increase), compared with vehicle. Acute ketamine injection concurrently increased γ power and locomotor activity in vehicle-treated rats, and these effects were attenuated in rats chronically treated with all three antipsychotics. The ability of haloperidol or clozapine to inhibit ketamine-induced elevation in γ power was not observed following acute administration of these drugs. These results indicate that modulation of γ power may be a useful biomarker of chronic antipsychotic efficacy.