THE mechanisms through which synaptic vesicle membranes are reinternalized after exocytosis remain a matter of debate^1–5. Because several vesicular transport steps require GTP hydrolysis^6–9, GTP-γS may help identify intermediates in synaptic vesicle recycling. In GTP-γS-treated nerve terminals, we observed tubular invaginations of the plasmalemma that were often, but not always, capped by a clathrin-coated bud. Strikingly, the walls of these tubules were decorated by transverse electron-dense rings that were morphologically similar to structures formed by dynamin around tubular templates^10,11. Dynamin is a GTPase implicated in synaptic vesicle endocytosis^12–14 and here we show that the walls of these membranous tubules, but not their distal ends, were positive for dynamin immunoreactivity. These findings demonstrate that dynamin and clathrin act at different sites in the formation of endocytic vesicles. They strongly support a role for dynamin in the fission reaction and suggest that stabilization of the GTP-bound conformation of dynamin leads to tubule formation by progressive elongation of the vesicle stalk.