Tracing of neuroanatomical pathways commonly involves the histochemical demonstration of horseradish peroxidase, using the chromogen tetramethylbenzidine. A new modification of this reaction using ammonium paratungstate stabilizer retains high sensitivity while permitting the reaction to be performed at pH 6.0 in isotonic solutions. The reaction product resists solvents, allowing Nissl-stained sections to retain their peroxidase labeling. With subsequent stabilization by diaminobenzidine, the tissue is suitable for electron microscopic study and is compatible with post-embedding immunocytochemistry.