The allergic inflammatory disorders of the nervous tissue are associated with a complex series of cellular and humoral immune activities and they usually result at least in demyelination, but according to morphologic evidence also in secondary neuronal changes. Using the colorimetric method of Ellman et al. (G. L. Ellman, K. D. Courtney, V. Anders, and R. M. Featherstone, 1961, Biochem. Pharmacol. 7:88-95) the activities of enzymes splitting acetylthiocholine iodide (AThCh) were determined from various parts of the somatic nervous system of rabbits with experimental allergic neuritis (EAN), a primary demyelinating disease of the peripheral nerves. It was found that the total activity of AThCh-splitting enzymes was decreased already in an early phase of the disease in the dorsal root ganglia (DRG). In a well developed phase of the disease the activity of acetylcholinesterase (AChE) seemed to be decreased by 33% in the ventral roots and by a lesser amount in the DRG and the most proximal part of the sciatic nerves. The mechanism of the recorded changes may be related either to specific or to nonspecific immune events or to both. Proteolytic activity released by macrophages in the target tissue may, by inactivating the hydrolytic activity of AChE, at least partly explain these findings. Because the activity of AChE in the structures studied derives from a neuronal origin, our results provide biochemical evidence for the involvement of neurons in the sensory ganglia and of axolemma in the ventral roots in EAN.