We have evaluated the efficacy of 18 cationic mitochondrial dyes that, as a class, show some ability to stain living nerve terminals. Several of these agents provide excellent staining of neuromuscular junctions in a wide range of species. More detailed studies of the most effective of these dyes--4-(4-diethylaminostyryl)-N-methylpyridinium iodide (4-Di-2-ASP)--indicate that it has no lasting effect on the structure or function of motor nerve terminals. As demonstrated in the accompanying paper (Lichtman et al., 1987; see also Lichtman et al., 1986), 4-Di-2-ASP can therefore be used to follow the configuration of identified motor terminals over arbitrarily long intervals.