The atomic force microscope (AFM) was used to directly image purified synaptic vesicles. Individual secretory vesicles (~50 nm diameter) were resolved with the AFM when imaged either dry or in solution. Vesicles were observed repeatedly for periods of greater than 2 h. To ask whether the AFM can detect structural change of vesicles the osmolarity of the bathing medium was reduced from 330 to 110 mOsm. Hypo-osmotic treatment caused an expansion and flattening of the vesicles. Thus, using the AFM it is possible to resolve individual vesicles and follow changes in vesicular structure. This opens the possibility that the secretory event can be reconstituted and visualized in vitro in order to elucidate the roles of synaptic proteins in synaptic transmission. © 1995 Academic Press. All rights reserved.