Serum response has been used as a model for studying signaling transduction for many biological events such as cell proliferation and survival. Although expression of many genes is up- or down-regulated after serum stimulation, the Notch effector Hes1 displays oscillatory response. However, the precise mechanism and biological significance of this oscillation remain to be determined. Here, we identified serum-induced ultradian oscillators, including molecules in Stat and Smad signaling. Stat and Smad oscillations involve activation of Stat3 and Smad1 and delayed negative feedback by their inhibitors Socs3 and Smad6, respectively. Moreover, Stat oscillations induce oscillatory expression of Hes1 by regulating its half-life, and loss of Hes1 oscillations leads to G(1) phase retardation of the cell cycle. These results indicate that coupled Stat and Hes1 oscillations are important for efficient cell proliferation and provide evidence that expression modes of signaling molecules affect downstream cellular events.