Brain Research, 54 (1973) 65-73 65 © ElsevierScientific Publishing Company, Amsterdam- Printed in The Netherlands HISTOCHEMICAL DETECTION OF ORTHOGRADE DEGENERATION IN THE CENTRAL NERVOUS SYSTEM OF THE RAT OSWALD STEWARD, GARY LYNCH AND CARL COTMAN Department of Psychobiology, University of California, Irvine, Calif. 92644 (U.S.A.) (Accepted October 25th, 1972) SUMMARY Following interruption of an afferent fiber projection in the rat brain, sites of terminal and possibly pre-terminal axonal degeneration accumulate formazan from iodonitrotetrazolium violet (INT). Increased formazan deposition first appears in sites of terminal degeneration several hours after the causative lesion, and persists for as long as 1 month. The effect has been observed in 7 different fiber systems, and thus appears to be a common characteristic of brain regions containing degenerating axon terminals. The method can serve as an adjunct to the classical silver techniques, and moreover has been found particularly useful for the localization of extremely minute brain lesions, such as the tracks by microelectrodes. INTRODUCTION If the axon of a neuron is severed, the axon and its synaptic terminals are isolated from the cell body, and consequently degenerate. This orthograde degenerative pro- cess, which ultimately results in the disappearance of the axon and its terminal rami- fications, is both interesting and useful as a neuroanatomical tool. After experimental interruption of a fiber system, the route of the fibers and their site of termination may be defined through the use of histological stains that more or less selectively demon- strate degenerating axons and their synaptic terminals. The Nauta 9, Nauta-Gygax 10, and Fink-Heimer 8 techniques are examples of such stains. We report here a new histochemical marker for degenerating nerve terminals that is easier and less time consuming than any technique described to date for the localization of terminal degeneration. The histochemical procedure results in massive deposition of a blue mono-tetrazolium salt at sites spatially comparable with terminal degeneration indi- cated by Fink-Heimer staining.