Transcranial direct current stimulation (tDCS) is a non-invasive technique that modulates excitability of neurons in the motor cortex. In particular, anodal tDCS leads to cortical facilitation and overall increase in excitability (Nitsche et al 2000). Although the effects of anodal tDCS on modulating motor cortex activation have been reported (Stagg et al 2009), there is little evidence of long term reduction of GABA concentration following anodal stimulation. Therefore, to characterize the duration of after effects of primary motor cortex (M1) excitability following anodal stimulation, we performed repeated magnetic resonance spectroscopy (MRS) measurements of GABA for 60 minutes after the stimulation.Eight right-handed healthy volunteers were recruited in RWTH Aachen University. All volunteers participated in accordance with the local ethics committee. Right handedness was assessed using the Edinburgh Handedness Inventory test (Oldfield, 1971). MRS scans were performed on a 3T Siemens PRISMA whole-body scanner (Siemens AG, Erlangen, Germany), equipped with a 20-channel receiver head coil. GABA-edited spectra were acquired from a volume of interest of 3x3x3 cm (27mL) carefully placed on the hand motor area of the left primary motor cortex [Fig 1 and 2]. MEGA-PRESS J-editing (Mescher M et al 1998), was used for GABA detection (TR\TE = 2000\68ms, 96 averages). Each acquisition took approximately 6:30 minutes. Data processing and quantification of spectra was performed with TARQUIN (Wilson M et al. 2011). An ANOVA was performed on percentage of GABA concentrations with stimulation (sham vs. anodal) and measurement (12 measurements: 2 pre-stimulation and 10 post-stimulation measurements taken every 6 minutes) as between- and within-participants factors. The main effects of stimulation and measurement s were significant F (1,6) = 10.157, p =.019 and F (11,66) = 3.031, p =.002. Percentage of GABA concentration was lower in the anodal compared to the sham stimulation (72% vs. 99%) and decreased across measurements. Crucially, the interaction was significant F (11,66) = 3.256, p =.001(Figure 3 and 4). GABA concentration decreased significantly in the anodal stimulation group at post-stimulation measurement 7 respect to pre-stimulation measurements, and then stayed the same at measurements 8, 9 and 10 ts(3)<3.799, ps<.038, whereas it showed no change in the sham group ts(3)<1. Our results revealed a significant decrease of GABA concentration following anodal stimulation that can be related to a decrease of activity of glutamic acid decarboxylase (GAD) 67 (Floyer et al. 2006). This after effect of anodal tDCS showed-up significantly after 68 minutes (at post-stimulation measurement 7) and remained stable in the following measurements (measurements 8-10). This result is consistent with the duration of previously reported behavioral effects of anodal tDCS over M1. Anodal tDCS demonstrated to be a promising tool to induce sustained GABA reduction in a non-invasive, painless, reversible and selective way.