Surveys methods for tracing neural connections with the tracer horseradish peroxidase. Discusses light-microscopic tracing following extracellular injections of tracer. Describes electron microscopic applications of horseradish peroxidase histochemistry for tracing neural connections, and compares various histochemical methods. Also discusses tracing connections after intracellular injections of HRP into physiologically identified neurons. Fully treats the relevant biological principles which guide each application