Vesicles bounded by single lipid bilayer membranes (outside diameter 350 to 400 A) can be made to enclose aqueous solutions of a variety of compounds. The procedure to prepare such vesicles in suspension is demonstrated with solutions of acetylcholine (ACh) in different concentrations as ‘inside’ media. Several points are submitted as proof that the vesicles can contain a particular compound not present in the ‘outside’ aqueous solution.