Dynamin is a large GTPase with a relative molecular mass of 96,000 (M-r 96K) that is involved in clathrin-mediated endocytosis and other vesicular trafficking processes(1,2). Although its function is apparently essential for scission of newly formed vesicles from the plasma membrane, the nature of dynamin's role in the scission process is still unclear(3,4). It has been proposed that dynamin is a regulator (similar to classical G proteins) of downstream effectors(5). Here we report the analysis of several point mutants of dynamin's GTPase effector (GED) and GTPase domains. We show that oligomerization and GTP binding alone, by dynamin, are not sufficient for endocytosis in vivo. Rather, efficient GTP hydrolysis and an associated conformational change are also required. These data argue that dynamin has a mechanochemical function in vesicle scission.