Susceptibility of various areas of the nervous system to TOCP (triorthocresyl phosphate) induced delayed neuropathy was assessed in groups of seven hens respectively, intoxicated with a single oral does of 500 or 1000 mg/kg body weight. 18 hens were used as negative controls. About 3 weeks after the treatment the hens were submitted to fixation by whole body perfusion and their nervous system processed either to paraffin sections stained with Bodian's silver stain and luxol counterstain, or to semi-thin plastic sections stained with toluidine blue. The examined areas were the cerebellum, the spinal cord at upper cervical, thoracic and lumbar level, the sciatic nerve, and the posterior tibial nerve. The extent of nerve fiber degeneration was assessed independently by two pathologists using a semiquantitative scoring system. The most susceptible areas were the cerebellum and the tibial nerve, followed by the upper cervical spinal cord. Within the cerebellum the nerve fibers in the rostral lobules, especially IV and Va, were affected. Whereas the resolution of plastic section was superior to that of paraffin sections in the cerebellum (mid-longitudinal level) and the spinal cord (transverse level), in the peripheral nerves the lesions were best recognized in the longitudinal, paraffin sections. There was very good agreement between both pathologists with respect to detection and grading of lesions in the most susceptible areas, but poor agreement in the areas of low susceptibility, indicating the danger of false results when lesions are not very distinct. In the susceptible areas the lesions induced with 500 mg/kg were sufficiently prominent, indicating that this dose-level is acceptable as positive control. In the hen nervous system, examination of the most susceptible areas, especially the rostral cerebellar lobules, appears to be suitable for detection of any kind of organophosphorus induced, delayed neuropathy.