Background <p>Altered glutamatergic and GABAergic signaling is increasingly implicated in the neurobiology of adolescent major depressive disorder (MDD). Transcranial magnetic stimulation (TMS) interventions may restore balance between excitatory and inhibitory neurotransmission. To explore these mechanisms, we quantified cortical GABA, glutamate, and glutamine concentrations with ultra-high field (7 T) magnetic resonance spectroscopy (MRS) in adolescents with MDD treated with TMS for six weeks.</p> Methods <p>7 T-MRS data were collected from the occipital lobe of adolescents with MDD (<i>n</i> = 26) prior to a TMS intervention and healthy controls (<i>n</i> = 20). Metabolite concentrations were compared between groups using ANCOVA, with age and sex as covariates. Participants with MDD were randomized to 1 or 10 Hz repetitive TMS (30 sessions) and a subsample (<i>n</i> = 17) underwent a second 7 T-MRS scan after the intervention. Changes in metabolite concentrations over six weeks were examined using linear mixed models with fixed effects for treatment, time, and treatment × time interaction, while controlling for age, sex, and number of prior failed medications.</p> Results <p>At baseline, ANCOVA showed no significant differences between MDD and healthy controls in occipital GABA, glutamate, or glutamine concentrations. In the MDD group, the mixed model revealed no significant effects of TMS frequency, time, or treatment × time interaction for any metabolite.</p> Conclusion <p>The findings revealed no baseline differences between adolescents with MDD and controls and no TMS-related changes in occipital metabolites. Future studies of GABA and glutamate neurophysiology in adolescents should focus on broader regional coverage and integration with complementary methods.</p> Trial registration <p>ClinicalTrials.gov: NCT03363919 registered on 30 November 2017 <a href="https://www.clinicaltrials.gov/study/">https://www.clinicaltrials.gov/study/</a> NCT03363919.</p>