The development and entrainment of fetal suprachiasmatic nuclei (SCN) are controlled by maternal cues, including hormones that cross through the placenta in a circadian rhythm. A recent study highlighted the effect of glucocorticoids (GC) on fetal SCN both in vitro and in vivo, where the application of the synthetic glucocorticoid, dexamethasone (DEX) in vivo regulated the c-Fos gene expression (Čečmanová et al., 2019). Using organotypic SCN explants from embryonic day 17 (E17) of a transgenic mPer2Luc mouse model, this research built on the initial study to further elucidate the action of GC upon in vitro application. Real-time recording of PER2-bioluminescence in E17 SCN explants confirmed that DEX increases the amplitude of E17 SCN explants, and DEX application at CT 15-18 leads to a phase advance of the rhythm. The specificity of the DEX effect was confirmed by application of the glucocorticoid receptor antagonist, mifepristone. Inhibition of the protein kinase A and C signalling pathways, which regulate c-Fos gene expression had no effect on DEX action in vitro in E17 SCN explants. No effect of DEX on PER2 protein turnover was observed. Using a newly optimized RNA isolation method followed by RT-qPCR, an increased level of c-Fos was detected in E17 SCN explants 1h after DEX application at CT...