Importance Growth hormone-releasing hormone (GHRH) has been previously shown to have cognition-enhancing effects. The role of neurotransmitter changes, measured by proton magnetic resonance spectroscopy, may inform the mechanisms for this response. OBJECTIVE To examine the neurochemical effects of GHRH in a subset of participants from the parent trial. DESIGN Randomized, double-blind, placebo-controlled substudy of a larger trial. SETTING Clinical research unit at the University of Washington School of Medicine. PARTICIPANTS Thirty adults (17 with mild cognitive impairment [MCI]), ranging in age from 55 to 87 years, were enrolled and successfully completed the study. INTERVENTIONS Participants self-administered daily subcutaneous injections of tesamorelin (Theratechnologies Inc), a stabilized analogue of human GHRH (1 mg/d), or placebo 30 minutes before bedtime for 20 weeks. At baseline and weeks 10 and 20, participants underwent brain magnetic resonance imaging and spectroscopy protocols and cognitive testing and provided blood samples after fasting. Participants also underwent glucose tolerance tests before and after intervention. MAIN OUTCOMES AND MEASURES Brain levels of glutamate, inhibitory transmitters γ-aminobutyric acid (GABA) and N-acetylaspartylglutamate (NAAG), and myo-inositol (MI), an osmolyte linked to Alzheimer disease in humans, were measured in three2 × 2 × 2-cm3 left-sided brain regions (dorsolateral frontal, posterior cingulate, and posterior parietal). Glutamate, GABA, and MI levels were expressed as ratios to creatine plus phosphocreatine, and NAAG was expressed as a ratio to N-acetylaspartate. RESULTS After 20 weeks ofGHRHadministration, GABA levels were increased in all brain regions (P < .04), NAAGlevels were increased (P = .03) in the dorsolateral frontal cortex, and MI levels were decreased in the posterior cingulate (P = .002). These effects were similar in adults with MCI and older adults with normal cognitive function. No changes in the brain levels of glutamate were observed. In the posterior cingulate, treatment-related changes in serum insulin-like growth factor 1were positively correlated with changes in GABA (r = 0.47; P = .001) and tended to be negatively correlated with MI (r = -0.34; P = .06). Consistent with the results of the parent trial, a favorable treatment effect on cognitionwas observed in substudy participants (P = .03). No significant associations were observed between treatment-related changes in neurochemical and cognitive outcomes. Glucose homeostasis in the peripherywas not reliably affected by GHRHadministration and did not account for treatment neurochemical effects. CONCLUSIONS Twenty weeks of GHRH administration increased GABA levels in all 3 brain regions, increased NAAG levels in the frontal cortex, and decreased MI levels in the posterior cingulate. To our knowledge, this is the first evidence that 20 weeks of somatotropic supplementation modulates inhibitory neurotransmitter and brain metabolite levels in a clinical trial, and it provides preliminary support for one possible mechanism to explain favorable GHRH effects on cognition in adults with MCI and in healthy older adults. TRIAL REGISTRATION clinicaltrials.gov Identifier: NCT00257712.