Tetramethyl benzidine (TMB) is a presumptively non-carcinogenic chromogen which yields a blue reaction-product at sites of horseradish peroxidase activity. Sixty-six distinct procedures were performed in rats and monkeys in order to determine the optimal incubation parameters for TMB. As a result, a procedure is recommended whose sensitivity greatly surpasses that of a previously described benzidine dihydrochloride method. Indeed, the sensitivity of this new method in demonstrating retrograde transport is markedly superior to that of the previously described benzidine dihydrochloride method. Furthermore, as a consequence of this enhanced sensitivity, many efferent connections of the injection site are also visualized. The injection site demonstrated by this TMB procedure is significantly larger than the one demonstrated when benzidine dihydrochloride or diaminobenzidine is used as a chromogen. Finally, this TMB procedure has been compared to two other TMB procedures and found to provide superior morphology and sensitivity.