The extracellular signal-regulated protein kinase ERK1/2 is a crucial effector linking extracellular stimuli to cellular responses: upon phosphorylation ERK [also known as mitogen-activated protein kinase P42/P44 (MAPK)] concentrates in the nucleus where it activates specific programs of gene expression. Notwithstanding the importance of this process, little is known about the modalities, time course and regulation of ERK exchange between nucleus and cytoplasm in living cells. We visualized the dynamic of nuclear translocation by expressing low levels (<150 nM) of fluorescently tagged ERK2 in living fibroblasts. Time-lapse imaging demonstrated that nuclear concentration can change bidirectionally with a time constant of a few minutes. The increase of nuclear concentration requires continuous MEK (also known as MAPK kinase) activity upstream of ERK and is rapidly reduced by the operation of phosphatases. We measured quantitatively the speed of ERK2 shuttling between nucleus and cytoplasm and determined that shuttling accelerated after ERK activation, becoming fast enough not to be rate-limiting for translocation. Finally, we demonstrated that ERK2 did not diffuse freely in the nucleus and that diffusion was further impeded after phosphorylation, suggesting the formation of complexes of low mobility. These results show that nucleocytoplasmic trafficking of ERK2 and its mobility are dynamically regulated in living cells.