We have developed a technique, called synaptic reconstruction, that permits nerve terminals of living vertebrate neuromuscular junctions (NMJs) to be isolated and then manually recombined with vacant endplate sites to form functional synapses. By reconstructing NMJs with various combinations of pre- and postsynaptic partners, or with varying degrees of pre- to postsynaptic alignment, the functional properties of the three anatomical components of the NMJ--nerve terminal, endplate, and the alignment between them--may be studied independently. Our experiments thus far indicate, surprisingly, that reconstructed NMJs function nearly normally. Thus, one feature of the intact vertebrate NMJ, precise alignment between presynaptic active zones and postsynaptic secondary folds, is either unnecessary for normal function or, alternatively, is spontaneously reestablished when an isolated terminal and vacant endplate site are placed in contact. We have also utilized synaptic reconstruction to examine a recently described property of NMJs: the regulation of quantal size among motor synapses in one muscle so that larger muscle fibers receive larger single quantal currents. Quantal size appears to be a postsynaptic attribute, suggesting that its regulation is achieved by a postsynaptic mechanism.