The Arthus reaction, passively induced in rat sciatic nerve by local injection of antibody and intravenous injection of antigen, was studied by immunofluorescence, using fluorescein and tetramethylrhodamine isothiocyanate. The reaction in nerve is similar to that occurring in skin. Antigen-antibody complexes formed at the site of the reaction activate complement and attract large numbers of polymorphonuclear leucocytes which then ingest the immune complexes. The significance of the Arthus reaction in relation to diseases of the peripheral nervous system is mentioned.