Many biological processes, such as circadian rhythms and somite segmentation [1], are regulated by molecular clocks. During somitogenesis, mRNAs for Notch signaling molecules, such as the Notch effector Hes1, oscillate periodically [1]. Here, we show that FGF stimulation induces the oscillatory expression of Hes1 in an ERK-dependent manner and also induces oscillatory activation of Ras and ERK activities. Our analysis demonstrates that oscillations in Ras/ERK activity require negative-feedback phosphorylation of Sos by ERK, suggesting that Ras/ERK oscillations could act as a novel molecular clock.