In this study we aimed to understand the relationship between stable individual differences in motor system excitability and reaction time by combining brain stimulation, neurochemical imaging, and behavioral testing. We used transcranial magnetic stimulation (TMS) to measure the intrinsic (resting) excitability of the pathway between the brain and muscles in the hand. We also measured reaction time by recording activity from the same hand muscle when it was used to respond during the performance of a simple computer task. We used magnetic resonance spectroscopy (MRS) to measure concentrations of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) in four different regions of the cortex. We found that individuals with a more excitable motor output pathway also had faster reaction times. Surprisingly, individuals with a more excitable motor output pathway also had higher concentrations of GABA in the motor cortex, but not in the other three brain regions we measured. These results suggest that people with a more excitable motor output pathway are faster to execute planned movements and also have more GABA available in the motor cortex. Larger amounts of GABA may support a greater capacity to inhibit a more excitable motor output pathway to maintain homeostasis within the motor system.