The INK4a-ARF locus encodes two distinct tumor suppressors, p16(INK4a) and p19(ARF). Whereas p16(INK4a) restrains cell growth through preventing phosphorylation of the retinoblastoma protein, p19(ARF) acts by attenuating Mdm2-mediated degradation of p53, thereby stabilizing p53. Recent data indicate that Mdm2 shuttles between the nucleus and the cytoplasm and that nucleo-cytoplasmic shuttling of Mdm2 is essential for Mdm2's ability to promote p53 degradation. Therefore, Mdm2 must export p53 from the nucleus to the cytoplasm where it targets p53 for degradation. We show here that coexpression of p19(ARF) blocks the nucleo-cytoplasmic shuttling of Mdm2. Moreover, subnuclear localization of Mdm2 changes from the nucleoplasm to the nucleolus in a shuttling time-dependent manner, whereas p19(ARF) is exclusively located in the nucleolus. In heterokaryons containing Mdm2 and p19(ARF), the longer the Mdm2 shuttling is allowed, the more Mdm2 protein colocalizes with p19(ARF) in the nucleolus, implying that Mdm2 moves from the nucleoplasm to the nucleolus and then associates with p19(ARF) there. Furthermore, whether or not Mdm2 colocalizes with p19(ARF) in the nucleolus, p19(ARF) prevents Mdm2 shuttling. This observation suggests that Mdm2 might be exported through the nucleolus and p19(ARF) could inhibit the nuclear export of Mdm2 by tethering Mdm2 in the nucleolus. Taken together, p19(ARF) could stabilize p53 by inhibiting the nuclear export of Mdm2.