We present a method that allows the on-demand release and mixing of zepto- to femtoliter vols. of solns. in the interior of vesicular nanoreactors. The reactors comprise a nested system of lipid vesicles, part of which release their cargo in the interior of the others during a thermotropic phase transition. The performance of individual reactors immobilized on glass is characterized using confocal microscopy and a fluorescent dye that reports diln. during the release. The results confirm the predicted temp.-induced response and reveal a release transition width of 3 DegC with a half time of .apprx.1 min. [on SciFinder (R)]