The molecular mechanism is also present in other neuronal and non-neuronal cells of the body, i.e., so-called peripheral oscillators. Importantly, the peripheral clocks are not directly synchronized by light but via by not yet fully recognized local signals and systemic signals derived from the SCN. Systemic signals set the phases of the peripheral oscillators relatively to the SCN by modulating the expression of some of the clock genes of the peripheral clocks. During ontogenesis, SCN morphology and rhythmicity develop gradually. In the rat, gestation takes 22 days and the nuclei are formed from the embryonic day 14 through E17 from the specialized zone of the ventral diencephalic germinal epithelium as a component of the periventricular cell groups. Whereas neurogenesis is completed at approximately E17, SCN morphology is not yet complete because the individual SCN neurons are not mutually interconnected by synapses. Despite its immaturity, intrinsic rhythmicity is already present in the SCN structure in late stages of embryogenesis. Clear day-night oscillation in metabolic activity was detected in the fetal rat SCN already from E19 through E21 by monitoring 2-deoxyglucose uptake. In contrast to previous studies, which used in situ hybridization to detect the transcript levels, in this study we used a more sensitive quantitative RT-PCR method. In the fetal SCN, most of the transcript levels were found to be relatively low and, therefore, there was a Xylitol possibility that their circadian variation could be missed especially in case when the background staining of the in situ hybridization probe was higher. Using the quantitative RT-PCR overcomes this methodological difficulty and provides more precise detection of daily variations in the transcript levels. The results revealed that in vivo, circadian vitexicarpin rhythms in gene expression are present in the fetal SCN before the functional molecular clock develops. Our results revealed a gradual development of circadian oscillations in individual clock gene expression in the SCN during late fetal development in vivo. These data suggest that the fetal clock does not operate at E19 but is functional at E21, immediately before parturition.