Recent studies have shown that biocytin may have multiple applications in neuroanatomical studies. Biocytin may be injected into the brain by iontophoresis or by pressure injection methods, and localized in tissue sections using avidin-conjugated labels. It is taken up by neurons and rapidly transported down axons in an anterograde fashion. Axons are completely labeled in a Golgi-like manner and can be examined at both light and electron microscopic levels. Biocytin can also be used in retrograde tract tracing experiments, although in some cases it appears that fibers must be damaged to produce such labeling. Retrogradely labeled cells may be completely labeled, resembling neurons stained with the Golgi technique. Individual neurons can also be labeled in a Golgi-like manner by uptake of biocytin from the extracellular space. Thus, it appears that biocytin is an especially versatile marker for neuroanatomical investigations.