Many human neurodegenerative diseases are characterized by the idiopathic death of cells narrowly restricted to a subset of neurons in a specific functional neuroanatomic system. Few in vivo models exist for the analysis of these types of degeneration. This report documents the death of sympathetic neurons resident in the superior cervical ganglia of rats after exposure to an exogenous chemical agent, the drug gnanethidine, as being mediated by natural killer (NK) cells. This is the first in vivo model of a disorder of the nervous system in which NK cells appear to be the principal effector cell, and thus could serve a central role in dissecting the normal and pathological function of NK cells. In addition, this pathogenetic mechanism appears to represent a novel type of autoimmune reaction that could have a direct bearing on a number of human illnesses.