Sprague‐Dawley rats were anesthetized and the mystatial vibrissae was injected with 0.3‐120 pmol of ¹²⁵I‐labeled affinity‐purified nonspecific rat IgG with and without additional unlabeled IgG in 50 μL of normal saline. Following a survival of 2 days, the rats were deeply anesthetized and perfused with saline and buffered formalin. The brains were removed and punch sample taken that included the facial nuclei. Uptake of radioiodinated IgG by the facial nucleus was determined by subtracting background activity, which was determined from the facial nucleus punch sample contralateral to the injection, from the ipsilateral facial nucleus. Uptake of radiolabeled IgG established a linear relationship with the amount of radiolabeled IgG injected, and only a very small amount was taken up (0.21 fmol per nmol injected). Addition of unlabeled IgG had no effect on the uptake of radiolabeled IgG. This evidence supports immunohistochemical evidence indicating that nonspecific serum IgG is taken up by motor neurons of the CNS with axons projecting the periphery. An increased understanding of this and other aspects of the relationship between the nervous system and the humoral immune system may provide insight concerning the pathogenesis of certain types of autoimmune nervous system disease.