The brain orchestrates the stress response, but it is also directly affected by it. We investigated grey matter volume (GMV) changes after acute psychosocial stress (Trier Social Stress Test) in humans. We found rapid brain plasticity after acute stress in several clusters including cingulate and insular cortices. The results also challenge the distinction between brain structure and function in T1-weighted images. ---------- Abstract and poster presented at [FENS2020][1]: **Rapid brain changes following acute psychosocial stress** **Aims**: The brain orchestrates the stress response, but it is also directly affected by it (McEwen, 1998). Stress rapidly alters brain function (i.e., activity and connectivity; Dedovic et al., 2009; Reinelt et al., 2019), but acute stress effects on brain structure have not been reported; despite evidence of regional brain volume changes after acute stress in animals (Gould et al.,1998) and with learning in humans (Taubert et al., 2016). We investigated grey matter volume (GMV) changes after acute psychosocial stress in humans and their relation to psychophysiological stress markers. **Methods**: 67 healthy men (25.8±2.7 years) underwent a psychosocial stress test (TSST, Kirschbaum et al., 1993) or a placebo version (Het et al., 2009) while blood, saliva, and psychometrics were sampled. T1-weighted MP2RAGE images (Marques et al., 2010) at 3T MRI (Siemens Verio) were acquired 30 min before and 45 min after the intervention. GMV changes were analysed using voxel-based morphometry (SPM12/CAT12). Associations with endocrine, autonomic, and subjective stress markers were tested with linear mixed models. **Results**: We found a significant group-by-time interaction in several clusters including anterior/mid-cingulate cortices and bilateral insula. There was no statistically significant association with stress-related changes in endocrine, autonomic, or psychometric values. **Conclusion**: We find rapid GMV changes following acute psychosocial stress in humans. Altered blood flow (Tardif et al., 2017), gliogenesis (Zatorre et al., 2012), and fluid redistribution (Streitbürger et al., 2012) could underlie GMV changes. The results also challenge the distinction between brain structure and function in T1-weighted images. [1]: https://forum2020.fens.org/