ABSTRACTIdentifying factors that influence the stability of DNA methylation measurements across biological replicates is of critical importance in basic and clinical research. Using a within-person between-group experimental design (n= 31, number of observations = 192), we report the stability of biological replicates over a variety of unique temporal scenarios, both in the absence and presence of acute psychosocial stress, and between individuals who have experienced early life adversity (ELA) and non-exposed individuals. We found that varying time intervals, acute stress, and ELA exposure influenced the stability of repeated DNA methylation measurements. In the absence of acute stress, probes were less stable as time passed; however, stress exerted a stabilizing influence on probes over longer time intervals. Compared to non-exposed individuals, ELA-exposed individuals had significantly lower probe stability immediately following acute stress. Additionally, we found that across all scenarios, probes used in most epigenetic-based algorithms for estimating epigenetic age or immune cell proportions had average or below-average stability, except for the Principal Component and DunedinPACE epigenetic ageing clocks, which were enriched for more stable probes. Finally, using highly stable probes in the absence of stress, we identified multiple probes that were hypomethylated in the presence of acute stress, regardless of ELA status. Two hypomethylated probes are located near the transcription start site of the glutathione-disulfide reductase gene (GSR), which has previously been shown to be an integral part of the stress response to environmental toxins. We discuss implications for future studies concerning the reliability and reproducibility of DNA methylation measurements.Abbreviations:DNAm – DNA methylation, CpG − 5’-cytosine-phosphate-guanine-3,’ ICC – Interclass correlation coefficient, ELA – Early-life adversity, PBMCs – Peripheral blood mononuclear cells, mQTL – Methylation quantitative trait loci, TSS – Transcription start site, GSR – Glutathione-disulfide reductase gene, TSST – Trier social stress test, PC – Principal component.