Some cationic mitochondrial dyes such as 4-Di-1-ASP and 4-Di-2-ASP stain presynaptic nerve terminals independent of neuronal activity. The photostable 4-Di-2-ASP dye, which is nontoxic to cells, has been employed to stain living nerve terminals in rabbit corneal epithelium, in rat epidermis and at mouse, snake and frog neuromuscular junctions, as well as to visualize the innervation of the human choroid and whole mounts of the gastrointestinal tract. Methods for using 4-Di-2-ASP to image neuronal cells in live animals have been described (Ex/Em = 488/607 nm).