In metazoa, the nuclear envelope breaks down and reforms during each cell cycle. Nuclear pore complexes (NPCs), which serve as channels for transport between the nucleus and cytoplasm, assemble into the reforming nuclear envelope in a sequential process involving association of a subset of NPC proteins, nucleoporins, with chromatin followed by the formation of a closed nuclear envelope fenestrated by NPCs. How chromatin recruitment of nucleoporins and NPC assembly are regulated is unknown. Here we demonstrate that RanGTP production is required to dissociate nucleoporins Nup107, Nup153 and Nup358 from Importin beta, to target them to chromatin and to induce association between separate NPC subcomplexes. Additionally, either an excess of RanGTP or removal of Importin beta induces formation of NPC-containing membrane structures--annulate lamellae--both in vitro in the absence of chromatin and in vivo. Annulate lamellae formation is strongly and specifically inhibited by an excess of Importin beta. The data demonstrate that RanGTP triggers distinct steps of NPC assembly, and suggest a mechanism for the spatial restriction of NPC assembly to the surface of chromatin.