The molecular mechanism of nuclear envelope (NE) assembly is poorly understood, but in a cell-free system made from Xenopus eggs NE assembly is controlled by the small GTPase Ran [1,2]. In this system, Sepharose beads coated with Ran induce the formation of functional NEs in the absence of chromatin . Both generation of Ran-GTP by the guanine nucleotide exchange factor RCC1 and GTP hydrolysis by Ran are required for NE assembly, although the roles of the GDP- and GTP-bound forms of Ran in the recruitment of precursor vesicles and their fusion have been unclear. We now show that beads coated with either Ran-GDP or Ran-GTP assemble functional nuclear envelopes in a cell-free system derived from mitotic human cells, forming pseudo-nuclei that actively transport proteins across the NE. Both RCC1 and the GTPase-activating protein RanGAP1 are recruited to the beads, allowing interconversion between Ran-GDP and Ran-GTP. However, addition of antibodies to RCC1 and RanGAP1 shows that Ran-GDP must be converted to Ran-GTP by RCC1 before precursor vesicles are recruited, whereas GTP hydrolysis by Ran stimulated by RanGAP1 promotes vesicle recruitment and is necessary for vesicle fusion to form an intact envelope. Thus, the GTP-GDP cycle of Ran controls both the recruitment of vesicles and their fusion to form NEs.